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MATERIALS OF MACHINES 



BY 

ALBERT W/ SMITH 

Director op Sibley College, Cornell Universitt 



SECOND EDITION, REWRITTEN 

FIRST THOUSAND 



NEW YORK 

JOHN WILEY & SONS, Inc. 

London: CHAPMAN & HALL, Limited 
1914 






Copyright, 1902, 1914 

BY 

ALBERT W. SMITH 



SlsT 



Stanhope jpress 

F. H.GILSON COMPANY 
BOSTON, U.S.A. 

NOV 18 1914 

©CI.A388444 




PREFACE TO SECOND EDITION 



The object of this book is to supply elementary 
knowledge of metallic materials used in the construction 
and operation of machines. The aim has been to bring 
together a group of correlated facts, to state them clearly 
and to show their relation. 

The book is in two parts: the first part deals with the 
manufacture of materials; and the second deals with the 
properties of materials. Understanding of the first part 
is believed to be an essential preliminary to the study of 
the second part, while understanding of the second part 
is very desirable for those who design, construct and 
operate machines. 

The writer wishes to express his great obligation to 
Professor Herman Diederichs for his ever-ready and 
efficient help on part first, to Professor George B. 
Upton whose researches and kind suggestions made it 
possible to present part second in its present form, and 
to Professor Heinrich Ries for his kindness in criticizing 
the chapter on Refractory Materials. 

Those who wish to follow part second with a fuller 
and more scientific treatment are referred to Professor 
Upton's forth-coming book on " Materials of Construc- 
tion." 

A. W. S. 
Ithaca, July 1914. 



ill 



TABLE OF CONTENTS 



Chapter Page 

I. Preliminary Consideration of Fuels 1 

II. Electric Furnaces 26 

III. Refractory Materials 30 

IV. Outline of the Metallurgy of Iron and Steel.. 37 
V. Outline of the Metallurgy of Copper, Lead, Tin, 

Zinc and Aluminum 77 

VI. Testing Materials 91 

VII. The Equilibrium Diagram of Iron and Carbon. . 110 

VIII. Cast Iron 121 

IX. Steel 144 

X. Heat Treatment of Steel 165 

XI. Non-Ferrous Alloys 177 

XII. Selection of Materials for Machines 193 



MATEBIALS OF MACHINES 



PART FIRST — MANUFACTURE 

CHAPTER I 

PRELIMINARY CONSIDERATION OF FUELS 

In general, a fuel is a substance which evolves heat 
while uniting chemically with oxygen. The fuels ordi- 
narily used, however, depend for their value upon the 
presence of carbon or hydrogen. A fuel may be pure 
carbon (solid), pure hydrogen (gas) or combinations 
of carbon and hydrogen like petroleum (liquid). Fuel 
may, therefore, be solid, liquid or gaseous. 

All industrial fuels have their origin in plant growth. 
A growing plant, because of energy received from the 
sun's rays, separates carbon dioxide of the atmosphere into 
its constituents, releasing the oxygen and storing the car- 
bon within the growing plant tissue; it also stores hydro- 
gen and oxygen derived from the sap so that the resulting 
plant fiber, wood or stems or leaves or grasses, consists of 
a combination of carbon, hydrogen and oxygen. 

Wood may be used directly as fuel or may be converted 
into charcoal. Nature's processes, acting through long 
periods of time, have converted plant fiber into coal or 
petroleum or natural gas; and artificial processes produce 
fuel gas and coke from coal, and produce fuel gas from 
petroleum. Hence, the potential heat energy of fuels is 

1 



2 MATERIALS OF MACHINES 

really energy which came from the sun and was stored 
through the agency of plant growth. 

The expression " complete combustion" means the com- 
bination of a fuel element with that amount of oxygen 
which produces the most stable compound. Thus, com- 
plete combustion of carbon produces carbon dioxide, 
C0 2 ; and complete combustion of hydrogen produces 
water, H 2 0, 

Combustion of unit weight of any fuel produces a 
definite quantity of heat which is called its calorific power. 
Since this is a quantity of heat, it is expressed in units of 
heat quantity; in this case British thermal units.* 

When carbon is burned with restricted oxygen supply, 
the gas carbon monoxide, CO, is formed. Under proper 
conditions of temperature and with adequate oxygen sup- 
ply, this carbon monoxide unites with more oxygen forming 
carbon dioxide, C0 2 , and evolving heat. Hence carbon 
monoxide is a gas fuel. 

Calorific powers of combustibles from experimental de- 
terminations are as follows: 

Combustible Calorific power 

Carbon burned to carbon dioxide (CO2) 14,540 B.t.u. 

Carbon burned to carbon monoxide (CO) 4,500 B.t.u. 

Carbon monoxide burned to carbon dioxide (C0 2 ) 4,300 B.t.u. 
Hydrogen burned to water (H 2 0) 62, 100 B.t.u. 

Marsh gas burned to carbon dioxide (C0 2 ) and 

water (H 2 0) 23,500 f B.t.u. 

* A British thermal unit (abbreviation, B.t.u.) is the quantity of 
heat necessary to raise the temperature of one pound of water from 
32° F. to 212° F., divided by 180. This gives the average quantity 
of heat per degree for the specified temperature limits. 

t Since one pound of marsh gas is composed of three-fourths 
pound of carbon and one-fourth pound of hydrogen, it would seem 
that its calorific power should be equal to f X 14,540 + \ X 62,100 = 
26,430 B.t.u. instead of 23,500 B.t.u. The difference, 2930 B.t.u., is 
energy used in separating the carbon and hydrogen of the marsh gas. 



PRELIMINARY CONSIDERATION OF FUELS 3 

The calorific power of carbon monoxide burned to car- 
bon dioxide may be derived from the calorific powers of 
carbon burned to carbon monoxide and carbon burned to 
carbon dioxide as follows : 

The carbon in one pound of CO equals, from the relation 
of atomic weights, M = f pound. In burning to CO, 
this f pound carbon evolved heat = 4500 X f = 1928 
B.t.u. If it had been burned to C0 2 , the heat evolved 
would have equaled 14,540 X f = 6231 B.t.u. The dif- 
ference between these two heat quantities equals the 
heat that would be evolved by burning carbon monoxide 
to carbon dioxide. This equals 6231 - 1928 = 4303 
B.t.u. This value checks closely with values derived from 
experiment. 

Temperatures resulting from combustion. — When a 
combustible is burned and the heat evolved is applied 
exclusively to the products of combustion, the tempera- 
ture attained depends on the following factors: 

1. The calorific power of the combustible. 

2. The nature, relative weights and the specific heats 

of the products of combustion. 

3. The quantity of air supplied. 

4. The temperature before combustion of the fuel and 

the air that supplies oxygen for the combustion. 

Factor 1 is a measure of the quantity of heat that is 
available for application to the combustion products, and 
with a given quantity of a given material to heat, the 
resulting temperature is proportional to the quantity of 
heat available. 

2. The temperature of combustion depends on the 
nature of the combustion products; this may be illustrated 
as follows : If the fuel is hydrogen, the combustion product 



4 MATERIALS OF MACHINES 

is water; the water must be vaporized, and the heat of 
vaporization becomes latent and hence cannot affect tem- 
perature. This is, of course, not true when there is no 
change of state of combustion product as in case of carbon 
burned to carbon dioxide. It is obvious that with a given 
quantity of heat available, the temperature increase de- 
pends upon the weight of substance heated, and upon the 
quantity of heat that will raise unit weight of the substance 
through a temperature range of one degree; that is, upon 
its specific heat. 

3. This factor affects the result because any excess of air 
increases the weight of matter to be heated by a given heat 
quantity with corresponding reduction of temperature. 

4. This factor affects the result because with 1, 2 and 3 
specified the products of combustion would be raised in 
temperature through a certain range and the final tem- 
perature would depend on the temperature of the sub- 
stance when heating began. 

To find the temperature produced by complete com- 
bustion of pure carbon without excess of air. — Chemically 
the combustion may be represented as follows : 

24 64 88 

C 2 + 20 2 = 2C0 2 . 

The relative weights appear above the symbols. For 
every 24 weight units of carbon 64 weight units of oxygen 
must be supplied; hence for one weight unit of carbon ff 
weight units of oxygen must be supplied. But the oxygen 
is supplied from the air, which is a mixture of oxygen and 
nitrogen with very small amounts of water vapor, carbon 
dioxide and other gases. For the present purpose all may 
be disregarded except nitrogen and oxygen which are 
present with close approximation as follows: nitrogen 
77 per cent by weight and oxygen 23 per cent by weight. 



PRELIMINARY CONSIDERATION OF FUELS 5 

Hence, to supply 23 weight units of oxygen, 100 weight 
units of air are necessary; therefore, it takes -^-3- weight 
units of air to supply one weight unit of oxygen. Hence, 
for complete combustion of one pound of pure carbon the 
weight of air required equals 

|| x Vs- = 11.6 pounds. 

The resulting gases, when combustion is complete, are 
nitrogen and carbon dioxide. The nitrogen takes no part 
in the combustion and the weight is 77 per cent of the 
air supplied or 11.6 pounds X 0.77 = 8.93 pounds. For 
every pound of carbon (see equation above) ff = 3.66 
pounds of carbon dioxide are produced. 

Summary. — One pound carbon (solid) burned in 11.6 
pounds air (gas) produces 8.93 pounds nitrogen (gas), 
and 3.66 pounds carbon dioxide (gas). 

Heat changes during this combustion. — Assume that 
the air supplied and the carbon are at a temperature of 
65° F. before combustion. Then, taking 0° F. as the 
heat datum, or temperature at which heat begins to be 
considered, the air would bring to the combustion 11.6 X 
65 X 0.237 = 178.7 B.t.u. in which 11.6 is the weight of 
air, 65 is the temperature range above the heat datum, and 
0.237 is the specific heat of air at constant atmospheric 
pressure and at a temperature of 65° F. The heat that 
one pound of carbon would bring to the combustion equals 
1 X 65 X 0.24 = 156.6 B.t.u. The complete combus- 
tion of the carbon would evolve 14,540 B.t.u. Hence, the 
total heat available to raise the temperature of the prod- 
ucts of combustion above 0° F. equals 14,540 + 178.7 + 
15.6 = 14,734 B.t.u. 

This heat would raise 8.93 pounds nitrogen and 3.66 
pounds oxygen to some temperature, t°, to be determined. 

The mean specific heat of nitrogen, with the temper- 



6 MATERIALS OF MACHINES 

ature range 0° to 4000° F., is 0.2848 * and of carbon dioxide 
for the same range is 0.2867.* 

The heat absorbed by the nitrogen while its temperature 
is raised to t° F. equals 8.93 X t X 0.2848 = 2.543 t; the 
heat absorbed by the carbon dioxide while its temperature 
is raised to t° F. equals 3.66 X t X 0.2867 = 1 .049 1 B.t.u. 

Hence t (2.543 + 1.049) = 14,734, 

whence f = 4100°F. 

The temperature produced by burning carbon monoxide 

gas may be found by the same method. This chemical 
combination is represented thus; 

56 32 88 
2 CO + 2 = 2 C0 2 . 

The weight of air per pound of carbon monoxide equals 
ft X J 2°3°- = 2.48 pounds. Of this air 77 per cent, or 1.9 
pounds is nitrogen; the resulting carbon dioxide equals 
If = 1.57 pounds. 

Summary. — One pound carbon monoxide (gas) burned 
in 2.48 pounds air (gas) produces 1.9 pounds nitrogen 
(gas) and 1.57 pounds carbon dioxide (gas). 

As before, assume 0° F. as a heat datum, and 65° F. 
as the temperature of the fuel and air supply before com- 
bustion. Then the air would bring to the combustion 
2.48 X 65 X 0.237 = 38.2 B.t.u. and the fuel would 
bring 1 X 65 X 0.245 = 15.9 B.t.u. The heat evolved 
by the combustion (calorific power of the carbon monoxide) 
= 4300 B.t.u. Hence, the total heat available to raise 
the temperature of the products of combustion above 0° F. 
= 4300 + 38.2 + 15.9 = 4354 B.t.u. 

* See " Experimental Engineering, " Carpenter and Diederichs, 
page 865. 



PRELIMINARY CONSIDERATION OF FUELS 7 

The mean specific heat of nitrogen (0° to 4000° F.) = 
0.2848, and of carbon dioxide = 0.2867. 

rriL ± rruurr 4.QQ7 TT 

inenc - ^ g x 0>2g48) + (L5? x 0-2 867) ~ 

The temperature produced by the combustion of 
hydrogen may also be found : The combustion is repre- 
sented chemically as follows : 

4 32 36 
2 H 2 + 2 = 2 H 2 0. 

For every weight unit of hydrogen 8 weight units of 
oxygen must be supplied, and the corresponding weight 
of air = Vk X 8 = 34.8 pounds. Hence, the combustion 
of one pound of hydrogen requires 34.8 pounds of air, of 
which 77 per cent, or 26.8 pounds, is nitrogen. The water 
resulting from the combustion = ^ 6 - = 9 pounds. 

Summary. — One pound hydrogen (gas) burned in 34.8 
pounds air (gas) produces 26.8 pounds nitrogen (gas) and 
9 pounds water (superheated vapor). 

The water produced is raised in temperature to 212° F.* 
and converted into steam which is superheated to the 
temperature resulting from the combustion. 

The specific heat of hydrogen at 65° F. = 3.37. 

The specific heat of air at 65° F. = 0.237. 

The specific heat of water at 65° F. = 1.00. 
The mean specific heat of nitrogen from 

0° F. to 4000° F. = 0.2848 
The mean specific heat of steam from 

212° F. to 4000° F. = 0.6724. 

The heat that is available to raise the temperature of 
the products of combustion may be found as follows : 

With 0° F. for a heat datum, and with 65° F. as the tem- 
perature of the hydrogen and air before combustion, one 

* Assuming the combustion to occur at atmospheric pressure. 



8 MATERIALS OF MACHINES 

pound of hydrogen would bring heat to the combustion 
equal to 1 X 65 X 3.37 = 219 B.t.u. The air would bring 
heat equal to 34.8 X 65 X 0.237 = 536 B.t.u.; the heat 
evolved by the combustion (calorific power of hydrogen) 
= 62,100 B.t.u. The sum of these values = 62,755 B.t.u. 
This heat, however, cannot all be applied to raise the tem- 
perature of the products of combustion, because the heat 
applied to vaporize the water does not affect temperature. 
This heat = 970 * X 9 = 8,730 B.t.u. Hence, the heat 
that really does affect temperature equals 62,755 — 8730 = 
54,025 B.t.u. This heat raises the temperature of 9 
pounds of water from 65° F. to 212° F.; it also raises the 
temperature of 9 pounds of steam from 212° F. to t°, the 
final temperature; it also raises the temperature of 26.8 
pounds of nitrogen from 65° F. to t°. Hence, the following 
equation may be written: 

54,025 = (212 - 65) 9 + (t - 212) (9 X 0.672) 
+ 0-65) (26.8X0.2848). 
Hence 13.68 1 = 54,025 + 1282.6 + 495.95 - 1323 

and * = ^^ = 3982°F. 

For the assumed conditions, then, the theoretical tem- 
peratures produced by complete combustion are: for 
carbon 4100° F.; for carbon monoxide 4397° F.; and for 
hydrogen 3982° F. 

It may seem strange that the combustion of carbon 
monoxide — which is partially burned carbon — should 
produce a higher temperature than the combustion of the 
original carbon; especially in view of the fact that the calo- 
rific power, or heat produced per pound, is 14,540 B.t.u. 
for carbon, and only 4320 B.t.u. for carbon monoxide. 
Inspection of the illustrative examples given above, how- 

* The heat of evaporation of steam at atmospheric pressure. 



PRELIMINARY CONSIDERATION OF FUELS 9 

ever, shows that while the heat evolved in the case of 
carbon monoxide is less, the weight of the products of 
combustion is less in greater proportion, and hence the 
resulting temperature is higher. 

The reasons why hydrogen, with its high calorific power, 
produces a temperature lower than either of the other 
fuels, are the greater relative weights of the substances 
heated, their greater heat capacity, and the absorption 
of heat for the vaporization of the water produced by the 
combustion with no resulting change of temperature. 

The theoretical temperatures found are never attained 
in actual combustion for the following reasons: 

1. Combustion is seldom complete; finely divided 
solid fuel falls through grates, and carbon monoxide and 
marsh gas often escape unburned to the stack, because of 
low temperature or insufficient oxygen supply; hence, the 
theoretical quantity of heat to raise the temperature is 
not completely evolved. 

2. In practice an excess of air is always supplied in 
the effort to prevent incomplete combustion, and this 
increases the weight of the gas to be heated and thus 
reduces the resulting temperature. For example, the 
temperature resulting from the complete combustion of 
carbon, with conditions as in the example on page 4 and 
with 50 per cent excess of air, is about 3000° F. instead of 
4100° F. with no air excess; with 100 per cent excess the 
theoretical temperature is about 2300° F. 

3. There are always radiation losses, which increase 
very rapidly with increase of temperature; while these 
losses may be much reduced by careful design and con- 
struction, and by the use of heat insulating materials, they 
cannot be reduced to zero. 

4. Moisture, which is usually present in the fuel and 
in the air supply, absorbs heat while it is heated, vaporized 



10 MATERIALS OF MACHINES 

and superheated, and this heat is taken away from the 
heat that is available to raise temperature. 

5. There is also a limit due to the fact that at high 
temperatures dissociation of the products of combustion 
may occur; this may be explained as follows: 

In a space like an ordinary furnace containing carbon, 
oxygen, carbon monoxide and carbon dioxide, there are 
probably two coexisting tendencies; one for carbon and 
oxygen to unite, and another for the combinations of 
carbon and oxygen to separate. At a given temperature 
these tendencies will be in equilibrium when a certain 
proportion exists among the substances present. As the 
temperature changes, however, the proportions corre- 
sponding to equilibrium change. When temperature rises 
in a space containing the substances specified above in 
equilibrium, some of the carbon dioxide will dissociate 
in order to restore the disturbed equilibrium. This dis- 
sociation is accompanied by absorption of heat which 
tends to check the rise in temperature and, therefore, 
to limit the temperature of combustion by the disso- 
ciation of combustion products. 

It is impossible in the present state of knowledge to 
state the proportions and temperatures quantitatively, but 
it is certain that in ordinary furnaces burning carbon fuel 
a temperature of 3000° F. can be produced, while it is 
probable that dissociation of C0 2 would prevent the 
temperature rising much above 3500° F. 

Similarly, highly superheated steam from the combus- 
tion of hydrogen would dissociate with rising temperature 
to restore equilibrium and would thus limit the tempera- 
ture of combustion of hydrogen to some value less than the 
theoretical value derived above. 

In engineering and metallurgical processes it is often 
required to produce temperatures higher than 3000° F. 



PRELIMINARY CONSIDERATION OF FUELS 11 

while guarding against incomplete combustion of carbon 
with a 50 per cent (or greater) excess of air, or while using 
a gas fuel diluted with a large proportion of nitrogen. 
This can be accomplished, up to the limit set by disso- 
ciation, by preheating the air supply, and the fuel also, 
if it is gas or vapor. 

With the method of computation used above it is found 
that in case of complete combustion of carbon with 50 
per cent excess of air, the preheating of the air supply to 
1200° F. increases the temperature of combustion from 
about 3000° F. to about 3900° F.; while with 100 per 
cent air excess and preheating to 1200° F. the theoretical 
temperature is about 3300° F. If the preheating is in- 
creased to 1500° F. with 50 per cent air excess, the resulting 
theoretical temperature becomes 4200° F., and with 100 
per cent air excess it becomes 3600° F. Of course, these 
values would be somewhat reduced by radiation loss. 
Since dissociation (see page 10) sets a probable limit at 
about 3500° F. it would seem, from the theoretical temper- 
atures just given, that preheating of air to produce high 
final temperature might be overdone. 

If pure oxygen were used instead of air for the support 
of combustion of carbon, the resulting theoretical tem- 
peratures would be higher, since there would be no nitrogen 
to heat. This temperature cannot be computed because 
of the uncertainty as to the value of the specific heat of 
carbon dioxide at very high temperatures. But in this 
case, as in those previously considered, the dissociation 
limit would be met, though its value might be changed 
because of the different composition of the products of 
combustion. 

It follows that if temperatures much above 3500° F. 
are required, they must be produced by other means than 
the burning of carbon fuel. 



12 MATERIALS OF MACHINES 

There are many other substances in nature which, 
when they combine with oxygen, produce temperatures 
higher than those that result from the burning of carbon 
or hydrogen. Two such substances, silicon and alu- 
minum, will be briefly considered for illustration. 

Silicon. — In the Bessemer process,* silicon, which con- 
stitutes only from 2 to 5 per cent of the charge at the 
beginning of the "blow," is burned to silica, SiC>2, and 
chiefly as a result of the heat thus evolved, the entire 
charge is raised in temperature from about 2500° F. to 
about 3500° F., at which the nearly pure iron is held in 
a fluid state. Obviously, if the silicon were present in 
larger proportion, its burning would produce a higher 
temperature, if the temperature were not reached at 
which vaporization of the silica or the iron would estab- 
lish a temperature limit. 

Aluminum. — Finely divided aluminum and iron oxide 
are intimately mixed, the mixture being called "ther- 
mit," and when this mixture is ignited the oxygen of the 
iron oxide goes over to the aluminum, forming alumina, 
A1 2 3 , and leaving pure iron. The chemical changes 
occur rapidly and very vigorously and the resulting tem- 
perature is said to be about 5000° F. At this temperature 
the alumina may be drawn off as a molten slag leaving 
the pure iron in a very fluid state. Sometimes the prod- 
uct of this process is used to mend cracked castings. 
The hot liquid iron from the process is allowed to run 
into the space between the cracked surfaces, the metal 
of these surfaces is melted, the space is filled with molten 
iron, which cools and solidifies, and the cracked surfaces 
are joined. 

This process is also applied to the production of metals 
like tungsten and chromium, in a very pure state, from 
* See page 60. 



PRELIMINARY CONSIDERATION OF FUELS 13 

their oxides. The metallic oxide is mixed with pure 
aluminum, both finely divided, and the mixture is ig- 
nited; the products are pure metal and aluminum oxide, 
the latter being removed as slag. 

The use of aluminum as a fuel is only justified by the 
production of exceptional results, because the fuel must be 
produced by an artificial and costly process. 

Nature does not produce silicon and aluminum fuel; 
nature's processes have produced vast quantities of 
silica, Si0 2 , and alumina, A1 2 3 , in which the combination 
with oxygen and evolution of heat has already occurred; 
that is, they are fuels that have been burned. This is 
true of almost all substances that might be used as fuel; 
in fact, it is true of carbon and hydrogen which occur 
in nature combined with oxygen as carbon dioxide, 
mechanically mixed with the air, and as water in its 
well-known distribution. But the energy of the sun's 
rays through the agency of plant growth is continually 
pulling away carbon from the oxygen of the carbon dioxide 
of the air and storing it with hydrogen and oxygen from 
the sap in the products of plant growth. In the past, 
these have been converted by nature's processes into 
coal, petroleum and natural gas and stored underground. 

Solid fuels may be classified as: 

(a) Raw fuels, such as coal and wood; 

(6) Artificial fuels, such as coke and charcoal. 

Raw fuels. — Coal is often classified as follows: 

(Lignite. 
Bituminous coal. 
Anthracite coal. 

Plant tissue is really converted into coal by gradual 
change; hence, each division of the classification covers 



14 



MATERIALS OF MACHINES 



a wide range and blends into the others. Description 
of coals is unnecessary here. 

The following table of percentage compositions shows 
the chemical changes which occur while plant tissue or 
woody fiber is changed to anthracite coal. 



Fuel 


Carbon 


H and in 
proportion to 
form water 


H available for 
combustion 


Wood 


48.5 
59.4 
65.0 
78.0 
94.0 


50.9 
39.0 
33.0 
19.0 
4.0 


0.6 


Peat 


1.6 


Lignite 


2.0 


Bituminous coal 


2.8 


Anthracite coal. 


2.4 







During this change, the percentage of available combus- 
tible increases, and the percentage of water to absorb 
heat decreases; hence, the temperature of combustion 
increases. 

Wood. — According to Professor Thorpe,* woody 
tissue, when freed from soluble and other foreign matter, 
has a percentage composition as follows: carbon, 48.5; 
hydrogen, 6.2; oxygen, 45.3. Since eight parts by 
weight of oxygen unite in combustion with one part of 
hydrogen, it follows that if the percentage of hydrogen 
present were 45.3 -h 8 = 5.6+, the oxygen and hydrogen 
would be present in just the right proportion to form 
water, and no hydrogen would be available for the evo- 
lution of heat. The amount of hydrogen really present 
is 6.2 per cent; and only the difference, 6.2 — 5.6 = 0.6 
per cent of hydrogen is available. This is practically 
negligible. Only 48 per cent of pure woody tissue, there- 
fore, is available for fuel. The temperature of combus- 

* "Coal: Its History and Uses," pp. 164-165. Edited by 
Professor Thorpe. Published by Macmillan & Co. 



PRELIMINARY CONSIDERATION OF FUELS 15 

tion is low for this reason, and also because the water 
resulting from the breaking up of the woody tissue, and 
that present as moisture, must be vaporized with absorp- 
tion of heat unaccompanied by rise in temperature. 
Therefore wood cannot be used directly as a fuel for the 
production of very high temperatures. 

Artificial fuels. Coke. — Bituminous coal, as shown 
in the foregoing table, contains carbon, hydrogen and 
oxygen. There is also a small amount of nitrogen present. 

When this coal is highly heated in a closed retort, 
destructive distillation takes place. The products of 
this process may vary with the time occupied, the tem- 
perature, the quality of the coal and other conditions, 
but in general are as follows : 

(a) Combinations of hydrogen and carbon in a very 
wide range of proportions, resulting in solid, liquid and 
gaseous hydrocarbons. 

(6) Combination of hydrogen and nitrogen into am- 
monia. 

(c) Combinations of hydrogen, nitrogen and carbon 
into aniline and many other compounds. 

(d) Combinations of carbon, hydrogen and oxygen 
into phenol and other compounds. 

(e) Combinations of carbon and oxygen into carbon 
monoxide and carbon dioxide. 

(/) Pure hydrogen. 

(g) A nearly pure residue of carbon which is called 
coke. 

When sulphur is present, sulphur dioxide and other 
compounds of sulphur and the other elements present are 
produced. 

Charcoal. — Wood may also be subjected to destruc- 
tive distillation, the process being essentially the same as 
that just described. The carbon residue is called charcoal. 



16 MATERIALS OF MACHINES 

The object of the processes for the production of coke 
and charcoal is to produce a concentrated fuel by removing 
all substances except the available fuel element, carbon. 
Obviously, this increases the temperature produced by 
combustion. 

Gaseous hydrocarbons, carbon monoxide and hydrogen 
are gas fuels which pass off, and hence, unless these are 
utilized, the process sacrifices a portion of the fuel in order 
to increase the temperature of combustion. 

Pulverized coal. — For certain service pulverized 
coal is used as fuel with great advantage. Bituminous 
coal is dried and ground so that about 90 per cent will 
pass through a screen of 100 meshes to the inch; this 
coal powder is blown into a furnace in a cloud where it 
burns while in suspension. The surface of contact of the 
coal with the oxygen of the air is vastly increased by 
pulverizing and it is unnecessary to supply excess of air 
as in burning coal in lumps; in fact, the air supply may be 
kept almost at the theroetical requirement in burning 
pulverized coal, with the result that the temperature 
produced approximates the temperature limit set by 
dissociation of carbon dioxide; because of this, it has been 
difficult to provide refractory lining that will withstand 
the temperature of combustion of powdered coal. This 
fuel requires a very large combustion chamber as the coal 
powder must be burned while in suspension and the 
carbon monoxide formed burns with a very long flame; for 
this reason, it has proved successful for use in the long 
rotary kilns used in the manufacture of cement, and it 
has failed as a boiler fuel, though it might possibly be 
used where the boiler type permits large combustion- 
space. The" temperature, also, is too high for boiler ser- 
vice, but this could be controlled by increase in the air 
supply. 



PRELIMINARY CONSIDERATION OF FUELS 17 

The pulverized coal under certain conditions forms 
an explosive mixture with the oxygen of the air, and some 
very disastrous explosions have resulted. The coal now 
is ground only as it is needed; it is never stored after 
grinding; and it is protected from the air while in transit 
to the furnace. 

Liquid fuels. — The liquid fuels of greatest impor- 
tance are petroleum and alcohol. 

Crude petroleum consists of a complex combination 
of hydrocarbons, with great variations according to its 
source, together with small and varying proportions of 
oxygen, nitrogen and sulphur. The constituent hydro- 
carbons of any crude petroleum vary in composition 
from C4H10 through a long series of proportions to C13H23 
with steadily reduced proportion of hydrogen, and with 
increased density and reduced tendency to vaporize. 
If the temperature of crude petroleum is steadily raised, 
the hydrocarbons distill off in an order determined by the 
proportion of hydrogen present; first gasoline of various 
grades, then kerosene of various grades, and then lubri- 
cating oils of various grades, leaving a residue of hydro- 
carbons that are solid at ordinary temperatures. There 
may be also a residue of coke, that is, carbon uncombined 
with hydrogen. 

Crude petroleum is used as a fuel in the furnaces of 
steam boilers and in several types of metallurgical fur- 
naces. Special burners are used and provision is made 
for, (a) preheating the oil to increase fluidity; (b) sup- 
plying the oil under pressure; (c) delivering the oil for 
combustion in a very fine spray by the use of steam or 
compressed air. The temperature limit is the same as 
for carbon and hydrogen in other forms; but the air to 
support combustion and the oil of the spray can be so 
intimately mixed that it is unnecessary to supply an 



18 MATERIALS OF MACHINES 

excess of air and hence, the theoretical limit of temperature 
can be approached more nearly, than with solid fuel. 

Alcohol is not yet an important industrial fuel, and it is 
not used at all for metallurgical purposes; but it may 
in the future become a very important factor in metal- 
lurgy and power development. 

The supplies of coal, oil and natural gas will eventually 
be exhausted, since there is continual draft on a store 
that is never renewed, and then it will be necessary to 
supply heat energy for human uses from the sun's energy 
stored by plant growth of the present time. As already 
stated, wherever plants grow, carbon is taken from the 
carbon dioxide of the atmosphere and hydrogen is taken 
from the water of the sap and they are stored together 
with oxygen as cellulose, starch or sugar. 

Cellulose, the chief constituent of plant fiber, and starch, 
the chief constituent of grains, potatoes, etc., are isomeric; 
that is, they contain the same elements in the same pro- 
portion, C 6 Hio05, but, probably because of some different 
arrangement of atoms, they are very different substances. 
Sugar, chemically C12H22O11, is found in sugar cane, sugar 
beets and other products of plant growth. Sugar or 
starch mixed with water and fermented with yeast yields 
a weak solution of alcohol which may be condensed by 
distillation, yielding ethyl alcohol, C 2 H 5 OH. In starch, 
C 6 Hi O 5 , hydrogen and oxygen are present in proportions 
to form water and hence only the carbon is available for 
fuel, while the water, since it must be heated, vaporized 
and superheated, would absorb a part of the heat of com- 
bustion, making it unavailable and thus reducing the 
temperature of combustion. This is also true of sugar 
and hence they are not good fuels. But the alcohol pro- 
duced, C2H5OH, has four atoms of hydrogen available 
for every two atoms of carbon and only one corresponding 



PRELIMINARY CONSIDERATION OF FUELS 19 

molecule of water to absorb heat. Hence, the process 
that has changed starch or sugar into alcohol has greatly 
increased the fuel value. 

When cellulose, C 6 Hio0 5 , is subjected to dry distilla- 
tion at high temperatures it yields liquid products from 
which methyl alcohol, or wood alcohol, CH 3 OH, may be 
separated. This change is also accompanied by an in- 
crease in fuel value. 

If lack of other fuel and reduced price of alcohol made 
it desirable ; alcohol could be burned effectively by the 
method used for burning crude petroleum, as a source 
of heat for metallurgical furnaces or for steam boilers. 
Alcohol could also replace the more volatile petroleum 
products in internal combustion engines. This would 
utilize energy coming from the sun year by year in the 
present, and so the draft would be on a store that nature 
continually renews. Hence, it offers a possible solution 
of the fuel problem of the future. 

Gas fuel has several advantages over solid fuel for 
many metallurgical processes. 

1. Inferior solid fuel may be used for the generation 
of the gas fuel. 

2. The furnace for the production of the gas may be 
at a distance from the furnace where the gas is used, the 
transfer being made through pipes with resulting saving 
of valuable space. 

3. Heat may be more easily applied uniformly over a 
given surface, or concentrated locally, with gas fuel than 
with solid fuel. 

4. The air which supports combustion can be much 
more completely mixed with the fuel, and therefore, the 
excess of air over that necessary for complete combustion 
is reduced to a minimum, with a resulting increase in 
temperature. 



20 MATERIALS OF MACHINES 

5. If the mixture of air and gas is properly regulated, 
there will be a complete absence of smoke and soot, and 
the latter will not be mixed with the material treated. 

Gas fuel may be either natural or artificial. Natural 
gas, like coal and petroleum, is a product of nature's 
processes acting through long periods of time upon prod- 
ucts of plant growth. Natural reservoirs of this gas 
are tapped by drilling, usually in petroleum regions, and 
are piped to places where the gas is used. Natural gas 
consists chiefly of marsh gas, CH 4 , with small amounts 
of other hydrocarbons, hydrogen and carbon monoxide. 
All of these constituents are combustible and hence, the 
heat value is high; from 800 to 970 B.tiu. per cubic foot 
of gas under standard conditions of pressure and tem- 
perature.* 

Natural gas is available only in a few limited localities; 
and, since it is a stored product of nature's very slow 
processes, the reservoirs are eventually exhausted. While 
it lasts it is a very valuable fuel. 

There are three processes for the production of arti- 
ficial gas fuels: 

1. Illuminating-gas process. — This process consists of 
the destructive distillation of coal containing a large per- 
centage of volatile matter. It is similar to the process 
for production of coke, with the difference that gas is now 
the product and coke the by-product. The composition 
of the gas varies with the fuel and with the conditions of 
operation, but the variation is not usually very great 
from the following analysis: 

Hydrogen, H 49 per cent by volume. 

Marsh gas, CH 4 34 per cent by volume. 

Carbon monoxide, CO ... 8 per cent by volume. 

Ethylene, C 2 H 4 4 per cent by volume. 

Benzene, C 6 H6 1 per cent by volume. 

* See "Gas Power" by Hirshfeld and Ulbricht, page 15. 



PRELIMINARY CONSIDERATION OF FUELS 21 

The gas also contains small amounts of incombustible 
nitrogen, carbon dioxide and water vapor. 

The flame from this gas is luminous because of the 
presence of the hydrocarbons, C 2 H 4 and C 6 H 6 . When 
these burn with restricted oxygen supply, carbon is sep- 
arated as a finely divided solid which becomes incan- 
descent and luminous at the flame temperature, and 
which burns to C0 2 on reaching the flame limit. When 
the oxygen supply is adequate, as in the Bunsen burner, 
combustion is complete, no solid carbon appears and the 
flame is not luminous. The non-luminous flame may, of 
course, be used for light with mantle burners. 

2. Water-gas process. — In this process steam is passed 
through a bed of incandescent carbon. The reactions 
are as follows: 

C 2 + 4H 2 = 2C0 2 + 4H 2 . 
2C0 2 + C 2 =4CO. 
CO + H 2 = C0 2 + H 2 . 

These reactions probably go on simultaneously, and when 
the process is properly regulated, the composition of the 
resulting gas is usually within the following limits: 

Carbon dioxide, C0 2 2 to 15 per cent by volume. 

Carbon monoxide, CO. . . 20 to 40 per cent by volume! 

Hydrogen, H 50 to 65 per cent by volume. 

Marsh gas, CH 4 4 to 8 per cent by volume. 

There is also present in some cases a small amount of 
ethylene, C 2 H 4 ; but this is not usually enough to make 
the flame luminous and hence, if the gas is to be used for 
illumination, it is passed through a second furnace where 
it takes up the vaporized hydrocarbons, C 2 H 4 and C 6 H 6 . 
The breaking up of the steam into its constituent hydrogen 
and oxygen absorbs heat, and this heat is just equal to 
that given out when the hydrogen of the gas is burned 



22 MATERIALS OF MACHINES 

again. Hence, there is no gain in heat from the hydrogen 
that comes from the steam; in fact, there is a loss per 
pound of steam corresponding to the difference in heat 
carried by a pound of steam as it comes to the water-gas 
furnace, and the corresponding pound of steam (super- 
heated) as it is produced in the furnace where the gas is 
burned. Hence, water gas in burning gives a little less 
heat than would result from direct burning of the coal 
used in the water-gas furnace; but the process produces 
a fuel of high combustion temperature having the advan- 
tages of the gaseous form, see page 19. 

3. Producer-gas process. — This process, the most im- 
portant to the metallurgist, consists of burning coal with 
incomplete oxygen supply. There are many forms of 
gas-producers with great variation in details; the prin- 
ciples of operation, however, can be explained by reference 
to the form shown in Fig. 1. It consists of a chamber, A, 
lined with fire-brick, and having a suitable grate at the 
bottom. Coal is introduced through a hopper, B, so 
arranged that communication with the air need not be 
made when the solid fuel is put in. Air is admitted 
through the grate, and at D there is a steam-blower used 
to force combustion and to introduce steam. The cham- 
ber is connected with the gas-flue by the passage C. 
The most rapid combustion occurs near the grate. Air 
passes through the grate and its oxygen combines with 
the incandescent carbon, forming carbon dioxide, C0 2 ; 
this in passing up comes in contact with more incandes- 
cent carbon where the air supply is limited and taking 
up more carbon becomes carbon monoxide which passes 
up into the chamber. In the upper part of the coal 
where the heat is less intense, the volatile constituents 
distill off; in fact, the action is the same as in illuminat- 
' ing-gas retorts with the production of hydrogen, hydro- 



PRELIMINARY CONSIDERATION OF FUELS 23 

carbons, carbon monoxide, etc. This leaves coke which 
descends slowly becoming incandescent and uniting with 
oxygen and carbon dioxide to form carbon monoxide. Also, 
steam from the blower passes through the grates with 
just the same result as in the water-gas process producing 




Fig. 1. 



hydrogen and carbon monoxide. Since this steam is de- 
composed into its constituents with absorption of heat, 
it follows that when the hydrogen burns again it can only 
restore a part of the heat it has received, and hence the 
introduction of steam does not add to the heat evolved 
in the furnace; in fact, it decreases it. But though it is 
not a source of heat, it gives a convenient auxiliary means 
of temperature control and also tends to prevent clink- 
ering. 



MATERIALS OF MACHINES 



An average of the resulting gases from this process is 
i follows : 

( CO ... 24. 2 per cent by volume. 
Combustible | H 8.2 per cent by volume. 



8.2 per cent by volume. 

I CH 4 .... 2.2 per cent by volume. 

T 1. x-T-i \ C0 2 4.2 per cent by volume. 

Incombustible -k T „* « , , 

IN 61.2 per cent by volume. 

Therefore, 34.6 per cent of this gas is combustible, 
while 65.4 per cent is incombustible, and hence its com- 
bustion temperature must be low. It would seem, 
therefore, that " producer gas " could not be used for 
high temperatures. It becomes available for this pur- 
pose, however, through the regenerative furnace, orig- 
inally invented by Messrs. Frederick and C. W. Siemens. 
The gas, instead of being admitted to the furnace directly, 
passes through a chamber, B (Fig. 2), filled with " chequer 




Fig. 2. 

work," i.e., full of small intricate passages, surrounded by 
refractory material suitable for storage of heat. The air 
also passes through a similar chamber, A, and meets the gas 
at C, the entrance to the hearth D, where the metal is 



PRELIMINARY CONSIDERATION OF FUELS 25 

treated. The air is admitted above the gas, so that, because 
of its greater specific gravity, it shall mix more completely 
with the gas. Combustion occurs at C, and the products 
of the combustion, heated to a temperature corresponding 
to the combustion temperature of the fuel, pass over the 
hearth where they fulfill their function in the treatment 
of the charge with loss of heat and reduction of tem- 
perature, and then pass on, still at very high tempera- 
tures, through the chambers A\ and Bi to the stack. In 
passing they heat up these chambers to their own tem- 
perature, if the process is sufficiently long continued. 
Then the connections are changed so that the gas comes in 
through B h and the air supply through A h and A and B 
are connected with the stack. The gas and air passing 
through the heated chambers have their temperature 
raised before combustion takes place; then the tempera- 
ture is still further raised by the combustion, so that the 
products of combustion now pass to the stack after use 
in the hearth through A and B until the temperature of 
these chambers is raised to the higher temperature. 
Then the connections are again reversed and the entering 
gas and air are heated to this higher temperature before 
combustion, and so on. It would seem that an indefi- 
nitely high temperature could be produced by this method, 
but it cannot because a limit, about 3500° F., is set by 
dissociation of C0 2 into carbon and oxygen at high tem- 
peratures with absorption of heat. See page 10. 



CHAPTER II 
ELECTRIC FURNACES 

Electric furnaces are considered here because of the 
bearing their use has on the temperatures attainable for 
industrial purposes. Other types of furnaces will be 
explained and illustrated in connection with their use for 
metallurgical purposes. 

Electric furnaces are usually classified as; arc, resist- 
ance and induction furnaces. 

Arc furnace. — If carbon electrodes, which have been 
brought into contact to complete an electric circuit in 
which a suitable current is maintained, are separated 
slightly, the carbon of the separated surfaces and the 
intervening air are heated by the passage of the electricity 
across the gap. As a result of this heating, the opposing 
surfaces of the electrodes first grow red and then white 
and some of the surface carbon is vaporized ; the resulting 
carbon vapor fills the gap, the electricity flows through 
the vapor, and the gap may then be increased, because 
the resistance of the carbon vapor is less than the resist- 
ance of the air. The electricity thus flowing raises the 
temperature of the ends of the electrodes and the inter- 
vening carbon vapor to incandescence and electrical 
energy is transformed into heat and light. The light 
may be used for illumination, as in the arc lamp, or the 
heat may be used as in the arc-type electric furnace. A 
limit is set to the temperature that can be produced in 
this furnace; this limit depends on the supply and dis- 
posal of heat. The heat supplied by transformation of 
electrical energy in the arc is (a) radiated to the furnace, 

26 



ELECTRIC FURNACES 27 

or, (6) applied to the vaporization of carbon, (a) raises 
the temperature of the contents of the furnace, but (6) 
disappears as sensible heat and, therefore, does not affect 
temperature. As the temperature rises the amount of 
carbon vaporized increases and, therefore, the amount of 
heat abstracted from the energy supply for this purpose 
increases and the heat left over to raise temperature 
grows less. A temperature would finally be reached at 
which an increase in heat evolved (by the increase in 
electrical energy in the circuit) would be met by an equal 
disappearance of heat to maintain increased vaporiza- 
tion of carbon, and with these conditions the temperature 
would reach a maximum. With an electric arc between 
carbon electrodes, the temperature probably may reach 
about 6000° F. and hence the electric furnace furnishes 
a much higher temperature than a furnace for the com- 
bustion of carbonaceous fuel. The vaporization of the 
substances treated in the furnace might also affect the 
maximum temperature. 

Resistance type. — In this type of electric furnace 
electrodes have their terminal surfaces separated by a 
considerable distance and the space between them is 
filled by some substance, "the resistor,'' that offers suit- 
able resistance to the passage of the electricity. When 
the electricity flows its energy is changed into heat and 
light in the resistor, and the heat is passed on to a sub- 
stance that needs to be heated for some useful purpose. 
The substance treated may itself form the resistor, wholly 
or in part. The temperature attainable in this furnace 
is the temperature at which there is equality between 
any increase in the heat supply due to increased flow of 
electricity in the circuit, and the corresponding increase 
in heat absorption by vaporization of the resistor or the 
substance treated. 



28 MATERIALS OF MACHINES 

Induction type. — The induction coil consists of two 
windings of insulated wire about a suitable soft iron core. 
When alternating-current electricity flows in one coil, 
called the primary, an alternating-current is induced in the 
closed secondary coil. The voltages of the primary and 
induced currents are nearly directly proportional to the 
number of turns of wire in the respective coils. Hence, 
if the primary has a large number of turns and the sec- 
ondary a small number of turns, a high voltage current 
in the primary would induce a low voltage current in 
the secondary. This is, of course, the principle of the 
alternating-current transformer. Now, if the material 
to be treated in the furnace can replace the secondary 
coil a current will be induced in it by the current in the 
primary coil, and if this current is suitable the required 
heating effect will be produced. In this type, if the sec- 
ondary coil is not closed a current in the primary would 
induce an electromotive force in the secondary, but 
electricity would not flow. In some cases, the material 
treated is a solid which is liquefied by heat from the cur- 
rent. In the induction-type furnace, the secondary is 
closed by this liquid and the closing cannot be effected 
until the solid is melted and the solid cannot be melted 
until the secondary is closed. Hence, in starting the 
furnace, it is necessary to introduce some other material 
until working conditions are established. This type is, 
therefore, better fitted for continuous than for interrupted 
service. The temperature is limited in this furnace 
exactly as in the others. 

Combination of types. — In the arc-type furnace the 
electrodes often project vertically downward into the 
furnace with suitable arcing distance between their ter- 
minal surfaces and the material treated; two arcs are 
thus formed and the electricity also passes through the 



ELECTRIC FURNACES 29 

material treated, which thus acts as a resistor. Hence, 
this type is really a combination of arc and resistance 
furnaces. The arc furnace is virtually a resistance furnace 
in which the vapor between the electrodes is the resistor. 
In the induction furnace heat is produced by resistance 
to the flow of electricity in material treated and hence, 
this type really uses a combination of the induction and 
resistance principles. 



CHAPTER III 

REFRACTORY MATERIALS 

Crucibles and the linings of furnaces, ladles and other 
apparatus for metallurgical purposes must be made of 
materials having suitable resistance to fusion, to change 
of form at high temperature and to wasting by chemical 
or erosive action; also these materials must be so consti- 
tuted as not to interfere with desired chemical changes, 
or to cause undesirable chemical changes, in the materials 
treated by the process. 

Acid, neutral or basic linings for furnaces. — Tem- 
peratures of incipient fusion of pure refractories are 
approximately as follows: 

Silica, Si0 2 (acid) 3200° F. 

Aluminum silicate, Al 2 Si 2 7 (neutral) . . 3300° F. 

Chromic oxide, Cr 2 3 (neutral) infusible * 

Carbon, coke or graphite (neutral) .... infusible 

Alumina, A1 2 3 (basic) 3600° F. 

Lime, CaO (basic) 4500° F. 

Magnesia, MgO (basic) 4500° F. 

This table shows silica to be the least satisfactory 
material for use as a refractory, considering only the tem- 
perature of fusion. But, because of other qualities, it is 
used very extensively where the temperature to be sus- 
tained is safely below fusion point. 

Lime, CaO, in contact with aluminum silicate at high 
temperatures yields calcium silicate and calcic aluminate, 

* This means infusible at the maximum temperatures now used 
in industrial processes. 

30 



REFRACTORY MATERIALS 31 

and the mixture is fusible at a relatively low temperature, 
probably about 2000° F. If magnesia is substituted for 
lime there is a similar reduction of fusion temperature. 
Hence, lime or magnesia would act as a flux upon alumi- 
num silicate; and, conversely, aluminum silicate would 
act as a flux upon lime or magnesia. Hence, wherever 
alumina, silica and lime are in contact, or wherever 
alumina, silica and magnesia are in contact, if the temper- 
ature is above 2000° F. fluxing will take place; that is, 
the material will melt. In certain cases this melting is 
desirable, as, for instance, when refractory alumina and 
silica are removed from the blast-furnace as a fluid slag 
by the fluxing agency of lime that is introduced for this 
purpose (see page 42). But furnace linings must not 
be fluxed away, and hence the coming together of alumina, 
silica and lime, or of alumina, silica and magnesia should 
be prevented where temperatures exceed 2000° F., if any 
one of the substances is an essential part of the furnace 
lining. Ferrous oxide, FeO, also acts as a flux upon 
aluminum silicate. 

If a metallurgical process that produces a slag contain- 
ing lime or magnesia or ferrous oxide is carried on in a 
furnace lined with material containing excess of silica, 
that is, with an acid lining, the lining will be fluxed 
away. Also, with a basic lining and an acid slag, the 
lining will be fluxed away. Hence, a process producing 
an acid slag should be carried on in a furnace having an 
acid lining, and a process producing a basic slag should 
be carried on in a furnace having a basic lining. 

A neutral lining would be best, because it would resist 
the fluxing action of either an acid or a basic slag; but 
with the neutral materials at present available, it is 
difficult to prevent destruction of the linings by other 
causes. 



32 MATERIALS OF MACHINES 

Kaolin is a fine white clay used as an ingredient of 
porcelain and other white ware. It consists of a mixture 
of hydrated aluminum silicate with other substances like 
hydrous aluminum oxides, feldspar, quartz and mica. 
It has the quality of becoming plastic when mixed with 
water, and it thus can be molded into required forms 
which may be dried at low or moderate temperatures and 
calcined at higher temperatures with the removal of the 
water of hydration and with partial fusion. There 
results a hard, strong, refractory material. 

Fire-clay is a clay that is capable of withstanding 
high temperatures, say a minimum of 3000° F. Fire- 
clay in addition to hydrated aluminum silicate usually 
contains varying amounts of lime, magnesia and iron 
oxide; these, through their fluxing action, increase the 
fusibility of the calcined material; and hence, the higher 
the temperature to be sustained the greater the need 
that these substances should be reduced to a minimum. 
Fire-clays vary greatly in composition and in physical 
properties, and hence must be chosen with great care 
according to the service required; whether it is resistance 
to fluxing or abrasion, or to the action of gases, or capac- 
ity for enduring temperature changes safely. During 
the calcining of fire-clay considerable shrinkage occurs, 
which may be accompanied by distortion and cracking. 
This shrinkage may be reduced by mixing coke dust, 
graphite or silica sand, or more commonly ground fire- 
brick or flint clay with the plastic clay. The fitness of 
clays for use in refractories for given service depends not 
Only upon the chemical composition, but also upon the 
physical condition and manner of burning. Thus density, 
porosity and condition resulting from variation of tem- 
perature and duration of the burning affect refractori- 
ness. 



REFRACTORY MATERIALS 33 

Silica, when pure, cannot be made into fire-brick or 
used in mass for furnace linings, because a binding material 
is necessary. Silica rock, however, often contains enough 
clay to serve for binding; and often a mixture of ground 
quartzite or silica sand with fire-clay is used. 

Silica bricks are also made by mixing fine silica with 
a very small proportion of lime and adding water until the 
mass is somewhat coherent, when it is molded under 
high pressure, dried and fired. During the firing the lime 
combines with a small part of the silica and with such 
small amounts of aluminum as may be present to form a 
fusible slag that acts as a binder for the silica. 

A natural or artificial mixture of a large proportion of 
siliceous material — usually not less than 90 per cent — 
with clay or lime or both is usually called gainster. 

Chromite, FeCr 2 4 , a double oxide of iron and chro- 
mium, is neutral and very infusible. It occurs in nature 
as " chrome ore," which contains also alumina, magnesia, 
lime and silica. Proportions vary, but are often about 

as follows : 

Per cent 

Chromic oxide 50 

Ferrous oxide 35 

Alumina 3 

Magnesia 4 

Lime 5 

Silica 2 

This being neutral would be almost an ideal material 
for furnace bottoms if it were not for the fact that, after 
it is molded into place, it is almost impossible to produce a 
temperature high enough to cause it to set thoroughly, 
and this leaves it liable to destruction by mechanical 
erosion. The fusibility of the material varies with the 
amount and proportion of substances other than chromic 



34 MATERIALS OF MACHINES 

oxide present, and when properly selected, it is a very 
valuable material for daubing and patching furnace 
linings. 

Carbon in the form of coke or graphite is used as a 
refractory material either in bricks or crucibles. But 
though carbon is neutral and infusible, it is such a strong 
reducing agent, that is, it has such a strong tendency to 
unite chemically with oxygen, that in the presence of 
oxygen or oxides of other substances carbon monoxide or 
carbon dioxide is formed and passes off as gas, thus 
eventually destroying the crucible or the furnace lining. 
Carbon, therefore, can only be used as a refractory where 
oxygen is excluded. 

In making carbon refractory forms, tar is sometimes 
used as a binding material for the coke dust or ground 
graphite. The mixture is made, formed and dried and 
then fired with exclusion of oxygen. The tar is coked 
and the product is practically pure carbon. 

Clay is also used as binding material for finely divided 
carbon. A mixture is made of coke or graphite, clay and 
water; the mixture is formed, dried and fired. Crucibles 
made by these methods are used for melting steel in the 
crucible process. 

Bauxite is a mixture of a large proportion of hydrated 
alumina A1 2 3 • 2H 2 0, with clay, silica, iron oxide and titanic 
oxide; another hydrated aluminum oxide, A1 2 3 • 3 H 2 0, is 
also often present. Bauxite is used for making basic re- 
fractory bricks for furnace linings. To be suitable for 
this purpose it should contain, after calcining, about 90 
per cent of alumina. There is a large amount of combined 
water in the raw bauxite, a probable maximum being 30 
per cent, and the removal of this water during calcining 
causes great shrinkage and molds must, therefore, be made 
larger than the required size of the finished bricks. These 



REFRACTORY MATERIALS 35 

bricks are expensive and it is difficult to attain a tempera- 
ture in firing them high enough to produce a bond that 
insures requisite strength. Except for these two objec- 
tions, bauxite bricks provide a very satisfactory basic 
refractory. 

Lime, CaO, is practically infusible, but when pure is 
not satisfactory as a refractory because of difficulty in 
getting it to bind, and also, because it slakes rapidly on 
exposure to the air at ordinary temperatures, forming 
calcium hydroxide, CaH 2 2 , which crumbles and wastes 
on reheating. Lime refractories, therefore, cannot be 
made up and held in stock. 

Magnesia, MgO, obtained by calcining magnesium 
carbonate, MgC0 3 , is probably the best material for basic 
furnace linings. Commercially this refractory is obtained 
from "magnesite," a mineral found in Styria and Greece. 
The calcined magnesite for " magnesite bricks " contains 
from 80 to 90 per cent of magnesia; ferric oxide, alumina, 
lime and silica are present in varying proportions. The 
plasticity of calcined magnesite depends on the temper- 
ature of calcining; if this temperature is about 1500° F. 
the product has a specific gravity of about 3, while higher 
calcining temperatures give a specific gravity of about 3.7. 
The former is plastic enough to mold under pressure, 
while the latter lacks plasticity. Magnesite bricks may 
be made of a mixture of from four to six parts of the 
heavier with one part of the lighter calcined magnesite 
with from ten to fifteen per cent of water. This mixture 
is then pressed into molds. 

The only objection to magnesia for basic refractories 
is that it is expensive, because magnesite is found only in 
few localities far away from metallurgical centers. For 
this reason "dolomite" or magnesian limestone is often 
used. This cheaper mineral, after calcining, contains from 



36 MATERIALS OF MACHINES 

50 to 60 per cent of lime, 30 to 40 per cent of magnesia 
and small amounts of silica, alumina and ferric oxide. 
Calcined dolomite is ground fine and, either with or without 
admixture of clay, is made plastic with water, formed and 
fired. Another method uses tar as a binding material 
for crushed, calcined dolomite. Plastic dolomite is ex- 
tensively used for daubing and patching. 



CHAPTER IV 

OUTLINE OF THE METALLURGY OF IRON 
AND STEEL 

Iron occurs in nature combined with many other sub- 
stances. The world's supply of iron, however, is ob- 
tained almost exclusively from the oxides Fe 2 3 and 
Fe 3 4 . Carbonate ores, FeC0 3 , are reduced before 
smelting by roasting to FeO; this FeO takes up more 
oxygen from the atmosphere, becoming Fe 2 3 . The 
formation of these iron oxides by the processes of nature 
was accompanied by evolution of heat energy. This 
energy per unit weight of oxide was definite in amount 
and independent of the method or time of formation. 
To separate the oxide again into its constituents an exactly 
equivalent amount of energy must be supplied. In brief, 
the separation is accomplished as follows: Heat energy 
is supplied to the iron oxide whereby its temperature is 
raised. The bond which holds the iron and oxygen 
together, whatever its character, is weakened. But this 
alone is insufficient in this case to cause separation. 
Therefore, the heating is caused to occur in the presence 
of carbon (solid) or carbon monoxide (gas). Either of 
these substances has greater affinity for oxygen at high 
temperature than the iron; hence, with the help of the 
heat, is able to pull away the oxygen from the iron oxide, 
forming C0 2 , which, being gaseous, passes off, leaving 
the iron. The heat energy that is effective to weaken 
the bond plus the energy expended by the carbon or 
carbon monoxide in pulling away the oxygen from the iron 

37 



38 MATERIALS OF MACHINES 

is exactly equal to the heat energy that was evolved by 
the original combination of the oxygen and iron into iron 
oxide.* The real process is much more complex because 
of circumstances now to be considered. 

Sources of iron. — Full consideration of the ores of 
iron is beyond the scope of this work.f 

Iron ores may be classified as follows: 

1. Magnetic oxide, or magnetite, Fe 3 04. 

2. Ferric oxide, or red hematite, Fe 2 3 . 

3. Hydrated ferric oxide, or brown hematite, limonite, 
bog ores, etc. 

4. Ferrous carbonate or spathic ore, FeC0 3 . 

These ores always carry other substances, and the pro- 
portions vary between wide limits. Sulphur and arsenic 
are often present, and these, with carbon dioxide and 
water, may be removed as vapor or gas at comparatively 
low temperatures by the process of calcining or roasting. 

For calcining or roasting the ore is piled in heaps out 
of doors, or charged into kilns, with fuel in proper amount 
mixed with it. The fuel is ignited, and the mass slowly 
heated. Water is driven off as steam. If the ore is car- 
bonate, FeC0 3 , the C0 2 is driven off, and the resulting 
FeO is changed to Fe 2 3 by combination with oxygen of 
the air. If any iron pyrites, FeS 2 , is present the sulphur 
is oxidized, passing off as S0 2 , while the iron is also oxi- 
dized, remaining as Fe 2 3 . Arsenic is oxidized and vapor- 
ized if present. By the process of roasting, the structure 
of the ore is made more open, and hence better fitted for 
smelting. 

* By the law of conservation of energy. 

f "Ore Deposits of the United States," J. F. Kemp; Scientific 
Publishing Company. "Iron (the Metallurgy of)," T. Turner; J. B. 
Lippincott Company. 



METALLURGY OF IRON AND STEEL 39 

When roasting is carried on in the kilns it is often a 
continuous process. The kiln is like a foundry cupola, 
much enlarged in diameter. The ore and fuel are charged 
in at the top, and the roasted ore is withdrawn from 
openings at the bottom. 

The process is now usually omitted for oxide ores, the 
roasting being accomplished in the top of the blast-furnace. 

The early methods for the production of iron were 
direct methods, i.e., the product was wrought iron, which 
had not passed through the intermediate state of cast iron. 

Chemically these methods were as follows: rich ore, 
Fe 2 3 or Fe 3 04, was charged with charcoal into a rec- 
tangular hearth, and air-blast was supplied. The coal 
was ignited and the oxygen of the air combined with the 
carbon of the fuel to form C0 2 , which passing on over 
more incandescent carbon was reduced to CO, which 
came in contact with the Fe 2 3 , when the following reac- 
tions took place : 

3 Fe 2 3 + CO = 2 Fe 3 4 + C0 2 . 

2 Fe 3 4 + 2 CO = 6 FeO + 2 C0 2 . 

6 FeO + 6 CO = 3 Fe 2 + 6 C0 2 . 

Metallic iron and C0 2 were, therefore, produced. The 
silica and alumina of the ore united with FeO formed in 
the process — see reactions above — to form a double 
alumino-ferrous silicate or slag, which is fusible at a low 
temperature, and which was partly drawn off, while the 
iron remained in the hearth a spongy mass filled with 
molten slag. The mass was then heated to a welding 
temperature and taken to a hammer or squeezer, where the 
slag was removed by impact or pressure, and the mass 
was welded into a bloom. 

The details of this process varied in different localities. 
Rich ore and charcoal for fuel were required, and there 
was great waste of iron in the slag. It was, therefore, a 



40 MATERIALS OF MACHINES 

very expensive process, and was not available for the pro- 
duction of large quantities of iron. 

Nearly all the iron used to-day is reduced from ore 
in the blast-furnace. Fig. 3 shows a vertical section of 
a blast-furnace. The height varies from 40 to 100 feet, 
and the diameter at M varies from 12 to 25 feet. The 
inside form varies with the kind of ore and fuel used, and 
with the pressure and quantity of air of the blast. 

A blowing engine supplies air, at a pressure of from 5 to 
15 pounds per square inch, to the large pipe, P, which 
surrounds the stack. At intervals of the circumference 
of this pipe smaller pipes convey the air to the tuyeres, 
T, T, which deliver it into the furnace. The oxygen of 
the air combines with carbon of the fuel and forms carbon 
dioxide, which is almost immediately reduced, in the 
presence of carbon with restricted oxygen supply, to 
carbon monoxide. There is a constantly ascending cur- 
rent of carbon monoxide and nitrogen through the con- 
stantly descending solid materials. 

The "bell," B, prevents the escape of gas from the top 
of the stack, and insures its delivery into the pipe, G. 
Solid materials to be introduced into the furnace are 
placed in the annular space above B, and the latter, 
which is controlled by power, is lowered periodically and 
the charge drops into the furnace. 

The function of the blast-furnace is to change iron ore 
into pig iron. 

Pig iron is iron carrying from 3 to 10 per cent of carbon, 
silicon, manganese, sulphur and phosphorus, either chem- 
ically combined or mechanically mixed. 

The blast-furnace, therefore, provides for: 

(a) The removal of volatile constituents of the ore. 

(b) The reduction of the iron oxide of the ore. 

(c) The removal of the solid earthy constituents of 
the charge. 



METALLURGY OF IRON AND STEEL 



41 




■^«iii 




42 MATERIALS OF MACHINES 

It also provides carbon, silicon, manganese, sulphur and 
phosphorus under proper conditions for absorption by 
the iron. 

In order that the earthy solids of the ore shall combine 
with the flux into a readily fusible slag, silica, alumina 
and lime must be present (see page 31). If the ore 
carries silica and alumina, lime will act as a flux, and it is 
supplied in the form of limestone. If an ore contains 
silica only, alumina may be introduced with lime; or, 
siliceous and aluminous ores may be mixed and fluxed 
with limestone. 

Chemical changes in the blast-furnace. — Ore, coke 
and limestone are charged into the top of the stack and 
descend slowly to the crucible, 0, at the bottom, with 
steadily increasing temperature. The ore is first roasted, 
and when the temperature has reached about 450° F., the 
reduction of iron oxide by carbon monoxide begins slowly 
and continues at an increasing rate until the temperature 
reaches about 1100° F., when the reduction is probably 
nearly complete. The ore has now become a sponge of 
metallic iron mixed with silica, alumina, etc. But at this 
latter temperature the flux, which is limestone, CaC0 3 , be- 
gins to give off C0 2 , and the lime, CaO, thus produced 
comes in contact with the silica and alumina of the re- 
duced ore, and they descend together until a temperature 
is reached at which they combine to form a fusible slag, 
which melts and leaves the iron sponge. 

Introduction of carbon. — In the meantime a deposi- 
tion of carbon upon the iron sponge has been going on. 
This may be explained as follows: when carbon mon- 
oxide passes over metallic iron at a temperature of about 
750° F., the carbon monoxide is decomposed, solid carbon 
is deposited, and carbon dioxide and ferrous oxide are 
formed. This is what occurs in the blast-furnace when 



METALLURGY OF IRON AND STEEL 43 

the temperature of about 750° F. is reached by the metallic 
iron sponge. Then, as the temperature rises, ferrous 
oxide thus formed is reduced again by carbon monoxide, 
or by solid carbon; the carbon dioxide passes on upward 
with the gas-current, and the iron sponge remains im- 
pregnated with carbon. As this passes down with in- 
creasing temperature, iron carbide is formed, which is 
fusible at a much lower temperature than pure iron; a 
temperature which is reached below M in the blast- 
furnace. Therefore, the descending iron carbide is raised 
to its fusion temperature and melts and falls into the 
crucible 0. 

Introduction of silicon. — At very high temperatures, 
in the lower part of the furnace, where carbon and silica 
and metallic iron are in contact, a portion of the silica is 
reduced, and the resulting silicon is taken up by the iron; 
the remaining silica goes out with the slag. This change 
is favored by (a) high temperature, (b) excess of silica in 
the charge and (c) deficiency of lime in the slag. Usu- 
ally not more than 5 per cent of the silica of the charge 
is reduced. 

Introduction of manganese. — The manganous oxide 
of the ore is not reduced by carbon monoxide, but is in 
part reduced by carbon at high temperatures, and the 
resulting manganese combines with the iron. The un- 
reduced manganous oxide passes into the slag. Certain 
ores, like New Jersey franklinite, contain very large 
proportions of manganous oxide, and the product of their 
smelting is spiegeleisen, or ferro-manganese, containing 
from 5 to 25 per cent manganese. 

Introduction of sulphur. — Only a small part of the 
sulphur in the charge, which is chiefly in the coke as iron 
sulphide, FeS, appears in the pig iron, the rest passing 
into the slag as calcium sulphide. The amount of sul- 



44 • MATERIALS OF MACHINES 

phur that the iron can take up depends upon the capacity 
of the iron itself for sulphur, and also upon the amounts 
of silicon and manganese present (see page 128). High 
furnace temperature tends to reduce the amount of sul- 
phur in the pig iron. Also basic slag, i.e., slag with excess 
of lime, combines readily with sulphur, thereby reducing 
the amount absorbed by the iron. 

Introduction of phosphorus. — The phosphorus of 
the charge is usually in the ore in the form of calcium 
phosphate. This is not changed by carbon monoxide. 
But when the part of the furnace is reached where the 
slag is formed, the calcium phosphate is reduced in the 
presence of solid carbon. The lime goes to the slag; 
the phosphoric anhydride is broken up with formation of 
carbon monoxide and iron phosphide. Practically all 
of the phosphorus of the charge appears in the pig iron. 

Descent of coke. — Coke is the fuel almost univer- 
sally used in " hot-blast " furnaces. As the coke descends 
it is dried and raised in temperature. It meets carbon 
dioxide, which may come from the reduction of iron 
oxide, or from the roasting of carbonate ore, or from the 
roasting of limestone. The carbon dioxide is reduced to 
carbon monoxide with absorption of heat. The result- 
ing carbon monoxide may take part again .in reduction, 
or, if it is near the top of the stack, may pass off unchanged 
with the gas-current. The coke may also supply part 
of the carbon for formation of carbureted iron, and it 
also helps in the reduction of silica and phosphoric an- 
hydride. When it reaches the vicinity of the tuyeres 
it burns to CO and evolves the heat necessary for the 
operation of the furnace. All of the carbon of the coke 
appears either in the pig iron or in the gases issuing from 
the top of the stack. 

The combination of iron, carbon, silicon, manganese, 



METALLURGY OF IRON AND STEEL 45 

sulphur and phosphorus is fusible at a temperature which 
is reached a little below M in the blast-furnace. Hence, 
fusion occurs and the melted substance falls into the 
crucible, 0, together with the fluid slag. The iron and 
slag separate because of their difference in specific grav- 
ity, the slag floating on the top. When a sufficient 
amount has accumulated, the slag is tapped out through 
the " cinder-notch," allowed to cool, and transferred to 
the " cinder dump." The iron is tapped out through a hole 
low down in the crucible and allowed to run out through 
properly formed sand channels, where it cools as pig iron. 

Fig. 4 gives Professor Diederich's diagrammatic sum- 
mary of blast-furnace operation. While this does not 
pretend to give exhaustively all changes that occur, it 
does give the changes that are fundamentally important. 
The figure is self-explanatory. 

Obviously, a continuous current of gas flows from the 
top of the blast-furnace stack. The chief constituents 
of this are nitrogen, carbon dioxide and carbon monoxide. 
When the furnace is properly regulated, the carbon mon- 
oxide equals about 25 per cent of the issuing gas. It is, 
therefore, a gas fuel. A portion of this is often burned in 
the boiler furnaces to make steam to run the blowing 
engines and other auxiliary machinery. In some modern 
blast-furnace plants the stack gas is used as a fuel in in- 
ternal-combustion engines which develop power for the 
entire plant. The rest of the stack gas is used for heating 
the blast. 

In early blast-furnaces the blast entered the furnace 
nearly at the temperature of the outside air. Cold blast 
is still used in furnaces for the smelting of some special 
grades of iron. 

Heating the blast on its way from the blowing engine 
to the tuyeres results in: 



46 



MATERIALS OF MACHINES 



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METALLURGY OF IRON AND STEEL 47 

Higher temperature (see page 11). 

Economy of fuel, because the blast is heated by the 
waste gas fuel from the top of the blast-furnace stack, 
and less fuel needs to be burned in the furnace to main- 
tain a given temperature. 

Increased capacity, because, since less coke is charged, 
ore and flux may take its place. 

Grayer pig iron. — The higher temperature in the hot 
blast-furnace favors the reduction of silica, and the pres- 
ence of silicon in the iron causes a large part of the carbon 
to crystallize out as graphite, i.e., it renders the iron gray. 

The first method of heating the blast was to pass it 
through cast-iron pipes, which were enclosed in a furnace 
and maintained at the highest temperature that is safe 
for the material of the pipes. The temperature, however, 
is only about 900° F. In order that a higher blast tem- 
perature may be reached, special hot-blast stoves have 
been designed. The Cowper type is shown in Fig. 3. 
It consists of a cylindrical shell of iron plates lined with 
fire-bricks. C is a combustion-chamber, and D is a 
chamber filled with " chequer work." The gas fuel from 
the top of the stack passes through the pipe G, the dust- 
separator H, and into the combustion-chamber at J". 
Here it meets air which enters at A, and combustion 
takes place. The heated products of combustion pass 
down through D and on through E to the chimney. This 
process is continued until the combustion-chamber and the 
chequer work are raised to the temperature of combus- 
tion. In the meantime the air from the blowing engine 
enters the other stove (which has been previously heated) 
at K, passes up through the chequer-work chamber, and 
down through the combustion-chamber. The air gains 
heat from the chequer work and is thereby raised to a 
temperature somewhere between 1000° and 1500° F. 



48 MATERIALS OF MACHINES 

It then passes through L and P to the tuyeres T, where it 
enters the furnace. When this stove is cooled so that the 
blast is insufficiently heated, properly arranged valves are 
changed, and the gas burns in the stove at the left, while 
the blast enters through the stove at the right. 

Since the action of the blast-furnace is continuous, 
there must be at least three stoves, so that any one may 
be put out of service for cleaning or repairs. 

There are still some blast-furnaces that use charcoal 
as fuel, with cold blast. The product is white iron. 
Because of the lower temperature in the furnace less silica 
is reduced, and less silicon is absorbed by the iron. Be- 
cause of the small amount of silicon the carbon combines 
with the iron, instead of separating as graphite, and the 
iron fracture is white. This iron is used for chilled car- 
wheels, malleable cast iron, etc. 

Dry blast.* — It has been known for many years that 
the variation of moisture in the air seriously affects the 
operation of the blast-furnace. The air-blast enters the 
furnace and strikes the white hot material of the descend- 
ing charge including the coke. The heat in this part of 
the furnace is increased by combination of oxygen of the 
air with carbon of the coke; but it is decreased by 
the sensible heat required to raise the temperature of the 
blast to the furnace temperature. If the air is free from 
water vapor, only nitrogen and oxygen (in its combination 
with carbon) absorb heat; but if the air carries water 
vapor this also has to be heated. Although the percent- 
age by weight of the vapor in the air is small, yet its heat 
capacity — specific heat — is more than twice as great 
as that of nitrogen, and hence, high humidity of the air 
may chill the furnace and interfere with satisfactory 

* See Journal Iron and Steel Institute, Vol. LXVI, 1904, Part II, 
page 274. 



METALLURGY OF IRON AND STEEL 49 

operation. Of course, with any amount of vapor the 
temperature in the furnace could be maintained if there 
were coke enough in the portion of the charge where the 
blast enters; but the water vapor in the air is subject to 
sudden and irregular variations, and it is impossible to 
adjust the quantity of coke at the tuyeres to meet these 
variations because of the long time required for the de- 
scent of the charge. The ideal condition is with air for 
the blast with a uniform minimum of water vapor. 

Mr. James Gayley designed a plant for furnishing 
" dry-air blast/' which was constructed and applied to 
the Isabella furnaces of the Carnegie Steel Company at 
Etna, Pa., and started August 11, 1904. Mr. Gayley's 
scheme is as follows: The air on its way to the blowing 
engine passes through a chamber containing coils of pipe 
that are kept at a low temperature through the agency 
of an ammonia-compression refrigerating plant. The 
moisture in the blast air is deposited on the coils as water 
or frost; and, after the accumulated frost has reached a 
certain thickness, the refrigerating medium flowing in the 
coils is shut off and a hot medium, which melts the frost, 
is forced through in its stead. The resulting water is 
drawn off, the refrigerating conditions are restored and 
frost begins to form again. The frost-melting process is 
applied to a few coils at a time and does not interfere 
with continuous running. 

The introduction of this dry-air blast makes it possible 
to increase the burden — the weight of ore per ton of 
coke — by an amount equal to 20 per cent or more with 
proportionate decrease in cost of fuel per ton of product. 
Also, since the air after passing the drying chamber 
reaches the blowing engines at a reduced temperature 
and increased density, the weight of air delivered per 
engine stroke is increased and the engine speed is reduced 



50 MATERIALS OF MACHINES 

with corresponding gain in power cost. In the experi- 
mental plant this gain in power in the blowing engines was 
greater than the power required to drive the refrigerating 
plant. But the greatest gain from Mr. Gayley's invention 
results from the power of control that it gives over con- 
ditions of operation. It becomes possible as never here- 
tofore to turn out different grades of pig iron at will, and 
thus to command the most profitable market. 

Pig iron from the blast-furnace goes either (a) to the 
foundry to be converted into castings, or (b) to the pud- 
dling mill to be converted into wrought iron, or (c) to 
the Bessemer mill to be converted into Bessemer steel 
or (d) to the open-hearth furnace to be converted into 
open-hearth steel. 

In the foundry pig iron is melted, with very little 
chemical change, and poured into sand molds, where it 
solidifies in the required form. This material is called 
cast iron. 

The pig iron is melted in a cupola-furnace. See Fig. 
5. This consists of a plate-iron shell lined with fire- 
brick and supported upon standards. Double doors, A, 
opening downward, are closed and held in position by a 
prop, P, and a sand bottom is rammed into place with a 
slope toward the tapping-hole T. The top of the cupola 
is open. 

A fan or blower supplies air-blast at a pressure of from 
5 to 10 ounces per square inch. The air enters through 
the pipe B and passes into the furnace by way of the cham- 
ber C and the openings E. 

The charge is elevated to a platform, indicated at F, 
and is introduced into the furnace through the charging- 
door D. Kindling and wood are first laid upon the sand 
bottom. Upon this the "bed " of coke is charged, and 
then alternate layers of iron and coke until the level of the 



METALLURGY OF IRON AND STEEL 



51 



charging-door is reached. The fire is lighted and the 
blast turned on. The coke burns, and the iron melts; 
and as the top of the charge settles gradually, more iron 
and coke are " charged on." 

The melted iron collects in the bottom and is drawn off 
periodically at T into a receiv- 
ing ladle, from which it is dis- 
tributed. Since the hot iron 
comes in contact with the 
air-blast there is always silica 
produced by the oxidation of 
some of the silicon. Also, a 
considerable amount of silica 
sand is introduced into the 
cupola adhering to the pig 
iron. If the cupola only runs 
one or two hours a day, as in 
small foundries, the silica does 
not interfere with operation. 
But for long or continuous run- 
ning it is necessary to include 
limestone with the charge for 
a flux, and to tap off slag at S. 

After all the iron to be melt- 
ed has been charged into the 
cupola, the drawing off of melt- 
ed iron continues and the 
charge settles down until the 
cupola is empty except for slag 
and a little iron. The blast is then stopped, the prop P 
is knocked out, the doors A swing down, and the residue 
of slag and iron drops out. 

Puddling process. — Both pig iron and wrought iron 
contain silicon, manganese, carbon, sulphur and phos- 




Fig. 5. 



52 



MATERIALS OF MACHINES 



phorus; but in pig iron the sum of these is usually from 
3 to 10 per cent, while in wrought iron their sum does not 
usually exceed 1 per cent. The object of puddling is to 
change pig iron into wrought iron. The process must, 
therefore, provide means for the removal of a part of these 
substances. The removal is effected by oxidation and the 
puddling process is carried on in a reverberatory furnace. 
This furnace requires description. 

See Fig. 6. A is a fire-box provided with a grate upon 
which solid fuel is burned. H is a hearth in which the 
metallurgical operation is carried on. E is a passage 




Fig. 6. 



connecting with the stack. F is the ash-pit, and B and D 
are doors for the introduction of fuel and the material 
to be treated in the hearth. The material in the hearth 
is heated by the hot gases which pass over it, and also by 
heat reflected from the highly-heated refractory material 
of the furnace roof. Solid fuel burns on the grate, and 
the air-supply through the ash-pit is under control. If 
air-supply were just sufficient for perfect combustion, 
the resulting carbon dioxide and nitrogen, at the temper- 
ature of combustion, would pass over the hearth and give 
up part of their heat to the furnace walls, and to the ma- 



METALLURGY OF IRON AND STEEL 53 

terial in the hearth, and then go on at lower temperature 
to the stack. But if air-supply is more restricted, carbon 
monoxide will result, which will burn in the hearth with 
air admitted above the fire or through the bridge-wall L. 
In this case the fire-box becomes a gas-producer, and the 
gas burns in the hearth. 

The flame which passes over the hearth of this furnace 
may be made an oxidizing, a neutral or a reducing 
flame. Thus, by a free admission of air through the fire 
or above it, complete combustion of all carbon monoxide 
is insured, and an excess of oxygen is carried over the 
hearth with the products of combustion. This results in 
a tendency to oxidize materials in the hearth. If the ad- 
mission of air is so regulated as to supply only just enough 
oxygen to complete the combustion of all carbon monoxide, 
the flame will be neutral, i.e., it will not tend either to 
give out or to take up oxygen. If the air-supply is re- 
stricted below the fire, carbon monoxide will result from 
the incomplete combustion; and if no air is admitted 
above the fire, this carbon monoxide will tend to take up 
oxygen from the materials in the hearth or to reduce 
them. 

In the form of reverberatory furnace used for puddling, 
the bottom of the hearth is made up of cast-iron plates 
which are covered to a depth of about three inches with 
a lining or " fettling " composed of silica and oxide of 
iron. The fettling is put in as follows : tap-cinder (which 
may be represented thus : 2 FeO, Si0 2 ) is charged in upon 
the iron plates, spread evenly, and subjected to a tem- 
perature high enough to soften it in the presence of oxygen. 
The FeO takes up oxygen and becomes Fe 2 3 . This will 
not remain in combination with the silica, and hence, the 
fusible silicate is converted into infusible ferric oxide and 
silica. Then scrap iron is charged in and subjected to 



54 MATERIALS OF MACHINES 

an oxidizing flame. It is thereby changed to magnetic 
oxide, which is raised to a welding heat and spread smoothly 
over the hearth bottom. 

If the hearth were lined with silica, the lining would be 
fluxed away by the ferrous oxide formed during the pud- 
dling process, with considerable loss of iron. Also, it is 
impossible to remove phosphorus in the presence of free 
silica.* 

There are two puddling processes: dry puddling and 
wet puddling. In the first, and less used process, white 
pig iron is heated in the hearth of the reverberatory fur- 
nace and subjected to the action of an oxidizing flame. 
White iron differs from gray iron in passing through an 
intermediate pasty condition before melting. During 
the passage through this condition, the iron is constantly 
stirred with a " rabble " or iron bar, which is inserted 
through a hole in the door D. The order in which oxida- 
tion of substances occurs is silicon, manganese, carbon, 
iron. A considerable part of the silicon and manganese 
is oxidized during the melting, and ferrous oxide also is 
formed. The silica and manganese oxide combine to 
form silicate of manganese, a fusible slag, and if silica is 
still left free it combines with ferrous oxide to form silicate 
of iron, also a fusible slag. When the silicon and manga- 
nese are completely oxidized, the oxygen attacks the car- 
bon and iron at the surface of the bath of metal. The 
resulting carbon dioxide passes off to the stack, and ferrous 
oxide acts as a carrier of oxygen, i.e., it is mixed with the 
bath and gives up its oxygen to combine with the carbon 
of the iron carbide, and the result is that carbon mon- 
oxide bubbles up to the surface of the bath and burns 
there to carbon dioxide, while the iron of the oxide and 
carbide remains in the hearth. This continues until the 
* See page 61. 



METALLURGY OF IRON AND STEEL 55 

carbon is almost entirely removed. Then, because of the 
raising of the fusing-point, the iron begins to solidify and 
is collected in a " puddle ball," which is really a sponge 
of iron with its interstices filled with slag. This is raised 
to a welding temperature, and put through a squeezer, 
where the slag is squeezed out and the iron is welded into 
a " bloom." This bloom is then put through a "roughing 
train" of rolls and is thereby converted into "muck 
bar," which is cut up, piled, reheated, welded under a 
hammer and rolled into "merchant bar." This piling, 
heating and rolling is sometimes repeated, with a result- 
ing product of finer fiber and increased strength and 
ductility. 

In wet puddling, the more commonly used process, 
gray iron is used, and it is allowed to become entirely 
fluid before it is "rabbled." The oxide of iron in this 
process, instead of being formed in the furnace, is de- 
rived from the fettling or is introduced in the form of 
"mill scale," * or from slag of previous heats that is rich 
in ferrous oxide or from some kind of rich ore. 

The chief distinction, then, between the two puddling 
processes is that in dry puddling the oxygen is supplied 
by the air, while in wet puddling the oxygen comes from 
the oxide of iron which is introduced with the pig iron. 

In order that the phosphorus may be removed it is 
necessary that there should be an excess of ferrous oxide 
in the fettling and the slag. Then phosphorus is oxi- 
dized to P2O5, and this combines with FeO to form ferrous 
phosphate, Fe 3 P20 8 . This is the form in which the phos- 
phorus appears in the slag. If there had been uncom- 
bined silica present in the slag the phosphoric anhydride 
would have been reduced again to iron phosphide and 

* The iron oxide that scales off from iron when it is hammered 
or rolled. 



56 MATERIALS OF MACHINES 

the phosphorus would have appeared in the iron instead 
of in the slag. 

Sulphur is removed in the puddling process, but the 
manner of its removal is not clearly understood. The 
sulphur exists in the pig iron as iron sulphide, and it 
appears in the slag in the same form. A basic slag (i.e., 
slag with excess of ferrous oxide), and a long period of 
contact of iron with the slag, are favorable to the removal 
of sulphur. 

A process is sometimes used which is intermediate 
between the blast-furnace process and the puddling 
process. It is called refining. It removes most of the 
silicon and manganese, but stops the process of removal 
before the iron becomes too infusible to be cast. The 
furnace for this process is a rectangular hearth with 
tuyeres on two sides bringing air under pressure. Melted 
iron from the blast-furnace may be run into this furnace 
and subjected to the oxidizing air-blast, or pig iron may 
be charged in with coke to melt it. In either case iron 
oxide may be introduced to hasten the removal of silicon 
and manganese. The iron, after completion of the treat- 
ment, is drawn out into sand molds, where it cools in 
the form of plates. These plates are broken up and 
converted into wrought iron in the puddling-furnace. The 
refinery changes gray pig iron into white pig iron, because 
it removes the silicon, which causes much of the carbon 
to change into graphite during cooling. 

Processes for making tool-steel from wrought iron. — 
The difference between wrought iron and tool-steel is 
chiefly in the amount of carbon contained. 

Wrought iron has from 0.1 per cent to 0.3 per cent. 

Tool-steel has from 0.5 per cent to 1.5 per cent. 
The change from wrought iron to tool-steel is, therefore, 
to be effected by addition of carbon. 



METALLURGY OF IRON AND STEEL 57 

Cementation Process. — Bars of very pure wrought 
iron, about f inch by 5 inches by 12 feet long, are packed 
in refractory boxes about 3 feet wide by 3 feet deep, 
with alternate layers of rather finely divided charcoal. 
These boxes, which are sealed up to exclude the air, are 
placed in a furnace, where the temperature is gradually 
raised to a maximum of 2000° F.; this temperature is 
maintained for several days, and then the furnace is al- 
lowed to cool down. Iron in contact with carbon at a 
high temperature tends to absorb carbon slowly, and it is 
found that the bars, after treatment as described, are 
changed to steel. The carbon, however, is not uniformly 
distributed, the structure is coarse, because of long-con- 
tinued high temperature, and the material is brittle. 
This material (called blister-steel) is changed to tool- 
steel by the crucible process, as follows: 

Crucible process. — The blister-steel is broken up 
into small pieces and charged into refractory crucibles 
about 2 feet high, with an average diameter of about 
10 inches. These crucibles are placed in a furnace, 
usually of the Siemens' regenerative type, where the 
melting temperature of steel can be attained, and their 
contents can be fused. This fluid steel is then cast into 
an ingot, which is homogeneous chemically, but of coarse, 
crystalline structure, because of its heat treatment. 
It is then reheated and hammered into standard sizes 
and forms, and the mechanical working gives it a fine 
homogeneous structure. 

The cementation process is often omitted and wrought 
iron is charged into the melting crucibles together with 
cast iron free from sulphur and phosphorus to furnish 
carbon. Coke or charcoal may be charged also to pre- 
vent oxidation at the surface, and to serve as a source 
of carbon. Some carbon may be absorbed from the cm- 



58 MATERIALS OF MACHINES 

cibles which contain either graphite or finely divided 
coke. Either ferromanganese or spiegeleisen * is intro- 
duced into the crucible, because the manganese reduces 
any iron oxide that may be present, and removes gas or 
causes it to go into solution, thus preventing porosity. 
The carbon of the ferro or spiegel increases the carbon 
of the steel. The melter regulates these sources of car- 
bon so as to insure close approximation to the required 
grade of the product. 

The Bessemer process. — Bessemer steel is very 
similar to wrought iron in chemical composition, but 
usually contains a little more carbon. The structure, 
however, is different, because of the difference in the 
method of manufacture. Thus wrought iron is built up 
from small particles of iron covered with slag. The slag 
is not entirely removed and the process of rolling draws 
out the particles into threads that are still surrounded by 
slag. This gives wrought iron the appearance of a fibrous 
structure. But Bessemer steel is cast into a solid ingot 
and then drawn down to the required shape and size. 
It, therefore, shows the crystalline structure of the iron 
itself. 

The Bessemer process changes pig iron into steel con- 
taining from 0.1 per cent to 0.6 per cent of carbon. This 
change is accomplished in a vessel called a converter. 
See Fig. 7. 

The vessel is made up of riveted iron or steel plates, 
and is lined with "ganister."f The converter is mounted 
upon trunnions A, A, and can be turned about the axis of 
the trunnions into any required position. Cold air from 
a blowing engine, at a pressure of from 20 to 25 pounds 
per square inch, enters at E, follows the passage shown 

* See page 43. 
f See page 33. 



METALLURGY OF IRON AND STEEL 



59 



to F, whence it passes into the converter through holes 
about f inch diameter that pierce the conical fire-bricks 
shown in the converter bottom. 

The Bessemer plant includes cupolas for melting the 
pig iron. The melted iron is conveyed to the converters 
either through properly-formed channels with refractory 
linings, or in ladle-cars running upon a track. Sometimes 




!: '!iW 

Mill 




Fig. 7. 



these cars transport the fluid iron directly from the blast- 
furnace to the converter. 

The converter is turned on its side, and a charge of 
iron is run in. It is then turned into a vertical position, 
a valve opens automatically to turn on the blast, and 
the air is forced through the bath of iron. The results 
are as follows: 

The oxygen of the air combines with the oxidizable 
substances of the bath ; and, iron being in excess, ferrous 
oxide is formed throughout the entire "blow." But 
ferrous oxide is reduced by silicon that is present with 



60 MATERIALS OF MACHINES 

formation of silica. Silica is also formed by direct 
combination of silicon with oxygen of the air. Manga- 
nese also is present and oxide of manganese is formed; 
this combines with silica to form silicate of manganese, 
a fusible slag. If the silica is in excess, some fusible 
silicate of iron is also formed. During this period bril- 
liant sparks of slag are thrown from the mouth of the 
converter. 

When all the silicon and manganese are removed, the 
carbon begins to be oxidized, directly by the oxygen of 
the air, and indirectly by the oxygen of the ferrous oxide. 
Carbon monoxide is formed, which passes off from the 
bath, and on reaching the mouth of the converter burns 
to carbon dioxide in a long flame. When the oxidation 
of the carbon is complete, there is no substance left to 
reduce the iron oxide formed, reddish fumes appear at 
the mouth of the converter, and the process^ is immedi- 
ately stopped by turning the converter on its side. 

The converter now contains nearly pure iron, and, 
although its fusion temperature is about 2900° F., it 
remains fluid. The fuel which, by its oxidation or com- 
bustion, has raised the temperature of the converter from 
the melting-point of pig iron to that of wrought iron, is 
the silicon, manganese and carbon of the pig iron. 

When the first experiments were made on the Bessemer 
process, it was thought that the blow could be stopped 
at the right point to leave the amount of carbon neces- 
sary to make steel; but it was impossible to get a uni- 
form product, and the resulting metal was brittle and 
worthless. 

This was because iron oxide remained in the metal, 
and because gas was occluded, causing porosity. To 
overcome these difficulties, the blow is continued until the 
carbon is completely removed, and a known proportion 



METALLURGY OF IRON AND STEEL 61 

of spiegeleisen or ferromanganese * is added to effect the 
recarburization. The manganese reduces the iron oxide, 
and, in some not very well understood way, removes the 
occluded gases or causes them to go into solution. After 
the addition of the spiegel or ferro, the steel is poured 
from the converter into a ladle, from which it is cast into 
ingots, which are rolled into rails, or plates, or into blooms 
which are to be rolled or forged into required forms. 

The basic Bessemer process. — During the blow as 
described, phosphoric acid and ferrous oxide are formed 
simultaneously, and these combine to form phosphate 
of iron, or ferrous phosphate; thus 3 FeO + P 2 5 = 
Fe3P20s. But this is reduced again to iron phosphide 
by silicon and carbon, and, therefore, little or no phos- 
phorus can be removed until after the complete removal 
of these substances from the metal in the converter. 
Ferrous phosphate is also reduced by silica, because the 
silica has greater affinity for ferrous oxide than phosphoric 
acid has, and so ferrous silicate is formed and phos- 
phoric acid is left, which is probably reduced to iron 
phosphide by the metallic iron, with formation of ferrous 
oxide. 

The lining of the Bessemer converter described is 
largely silica, and, therefore, silica is always present, and 
no phosphorus can be removed in a converter with a 
ganister or acid lining. It is necessary, therefore, to use 
for this process pig iron which is very low in phosphorus, 
since the presence of phosphorus in the product in any 
considerable amount is very undesirable. 

The fact that a large proportion of the iron ore of the 

world contains phosphorus, which is not removed in the 

blast-furnace, made it desirable to find a way to eliminate 

phosphorus in the steel-making process. This led to the 

* See page 43. 



62 MATERIALS OF MACHINES 

invention of the basic Bessemer process, in which a lining 
of lime and magnesia is substituted for ganister in the 
converter. The only free silica, then, is that which 
results from the oxidation of the silicon in the pig iron. 
This combines with lime, which is charged into the con- 
verter before the blow, and forms a stable slag, the silica 
being thereby rendered powerless to reduce the ferrous 
phosphate. 

The lime or magnesia present then replaces the ferrous 
oxide of the ferrous phosphate, forming calcium or mag- 
nesium phosphate, which is probably the form in which 
the phosphorus chiefly exists in the slag. 

In the acid process iron is not usually used which con- 
tains less than 2 per cent of silicon, because the combustion 
of at least that amount of silicon is necessary to produce 
a sufficiently high temperature in the converter. 

In the basic process silicon is an undesirable element, 
since all the silica produced must be neutralized by lime, 
in order that the process shall succeed. For this reason 
iron with 0.5 per cent silicon is best, and 1.5 per cent is 
the highest allowable limit. This makes it necessary to 
substitute some other fuel, and, therefore, pig iron high 
in manganese is used. The phosphorus, usually present 
from a minimum of 1.5 per cent to 3 per cent, is also a 
fuel and raises the temperature during the "afterblow." 
In the basic process little or no phosphorus is removed 
until after the complete removal of the carbon, and the 
blow has to be continued after the " dropping" of the 
carbon flame. The duration of the afterblow is deter- 
mined from a knowledge of the amount of phosphorus in 
the pig iron used, or by taking samples at intervals dur- 
ing the afterblow and making physical tests. 

The steel after the afterblow of the basic bessemer 
process must not be recarburized in the converter, be- 



METALLURGY OF IRON AND STEEL 63 

cause, in the presence of the spiegeleisen or ferromanga- 
nese, the phosphorus compounds of the basic slag tend 
to be reduced, the phosphorus liberated returning to the 
iron. Hence, the iron and slag in the converter are sep- 
arated as completely as possible by (1) pouring off the 
liquid slag from the iron and then (2) pouring the iron 
into a ladle, leaving the partly solidified basic slag in 
the converter. Then the fluid iron in the ladle freed from 
slag is recarburized in the usual way by the addition of 
spiegeleisen or ferromanganese. The iron is oxidized 
to a greater extent in the basic than in the acid blow,* 
and hence, more manganese is required to deoxidize the 
iron. 

The best pig iron for the basic process contains : 

Per cent 

Phosphorus about 3 

Manganese over 2 

Silicon about 0.5 

Sulphur less than 0.1 

This is white iron, because of high manganese and low 
silicon, whereas the high silicon iron used in the acid 
process is gray. 

Control of temperature in the Bessemer converter. — 

Either too high or too low temperature of the steel at 
pouring results in porosity, and, therefore, this temperature 

* Possibly because in the acid process the time for stopping the 
blow is indicated definitely by the appearance of the red fumes of 
iron oxide, whereas the time for stopping the blow in the basic process 
can only be determined by the knowledge of the amount of phos- 
phorus in the charge and knowledge of the rate of removal, checked 
by tests of the bath for phosphorus. Hence, the tendency to over- 
blow in the basic is greater than in the acid process, with the proba- 
bility of a greater amount of free oxygen and iron oxide in the blown 
metal. 



64 MATERIALS OF MACHINES 

must be carefully regulated. If iron too high in silicon 
is used in the acid process, too high temperature results, 
and conversely. 

In the basic process the difficulty is usually to keep 
the temperature high enough. If the temperature is too 
high, it may be reduced by charging in scrap-steel from 
the mill, which is thus remelted, absorbing surplus heat, 
and is rendered available for use. The temperature is 
also sometimes reduced by admitting a small amount of 
steam into the blast-pipe. 




Fig. 8. 

If the temperature becomes too low, the converter may 
be inclined, as shown in Fig. 8, during the burning out 
of the carbon. When the converter is vertical the carbon 
monoxide formed burns at the mouth of the converter, 
and the heat evolved is lost as far as raising the tempera- 
ture inside of the converter is concerned. In the in- 
clined position, however, a part of the air of the blast 
passes through the metal bath and forms carbon mon- 
oxide, and a part passes through the uncovered tuyere 
holes and furnishes oxygen to the carbon monoxide, and 
carbon dioxide is formed; i.e., combustion occurs inside 



METALLURGY OF IRON AND STEEL 



65 



of the converter, and the heat developed raises the tem- 
perature of the metal bath. 

Graphical representation of the basic Bessemer proc- 
ess. — Fig. 9 is copied from Wedding's ''Basic Bessemer 
Process," * page 143. The diagram is plotted from the 
results of experiments and shows the history of a blow 
in a basic converter. Horizontal distances from rep- 
resent time, each division corresponding to one minute. 
Vertical distances from represent percentages of the 
substances to be removed. Therefore, the curves represent 
the change in percentage of the substances during the 
blow. 

The silicon is reduced very rapidly from 1.2 per cent at 
the beginning of the blow, and after six minutes only 





] 


, i 


j < 


S i 




i 


' 


r 


TIME, MINUTES.. 
$ 9 10 11 12 13 14 15 16 17 18 ' 19 20 














































3.2 
3.1 

3.0 












































































































































\ 






























2.8 
2.7 
2.6 
2.5 
2.4 
2.3 
2.2 














\ 












































\ 










































\ 










































\ 












































vz 










































\ 










































_s 


























2.1 
2.0 
1.9 
1.8 
1.7 
1.6 
1.5 
1.4 
1.3 
1.2 
1.1 
1.0 
0.9 
0.8 




-v. 
















V 










































\ 










































\ 












\ 
































\ 










\ 
































\ 












\ 
























\ 












\ 
































. 










\ 
































\ 










\ 










Si 






















s 












\ 








\ 


































V 










^ 






















\ 










\ 








Mn 


s'V 






















\ 










\ 










\ 






















\ 








\ 












SJs 






















s 










\ 






0.6 
0.5 
0.4 
0.3 
0.2 
0.1 









v 




















\ 










\ 












S 






















\ 








V 












\ 






















\ 








\ 














\ 




\ 
















V 








\ 














\ 


\ 


















V 


s 








V 


J 
































-^ 








sr 


r 



Fig. 9. 



* Translated by Phillips and Prochaska; E. & F. N. Spon, pub- 
lishers, London. 



66 MATERIALS OF MACHINES 

0.1 per cent remains. From this point on the silicon is 
slowly reduced to zero. 

The manganese is reduced less rapidly than the silicon, 
changing from 1.05 per cent at the beginning to 0.15 
per cent after nine minutes. It remains nearly constant 
during the carbon reduction, and then becomes less than 
0.1 per cent. 

There is but little change in the carbon until most of 
the silicon is removed, when the curve drops rapidly, 
and the removal is practically complete in sixteen min- 
utes. Up to this time there has been little change in the 
phosphorus. This is, of course, because the ferrous phos- 
phate is reduced by carbon. From this point the removal 
of phosphorus is very rapid, being practically complete 
after the blow has continued twenty minutes. 

The curve of sulphur was shown on the original diagram, 
but it was not copied, as the quantity of sulphur remained 
nearly constant, its value being less than 0.1 per cent. 
The blow ends at the twenty-minute line, and the curves 
beyond show the effect of introducing spiegeleisen. 

Fig. 10 * gives the history of an acid Bessemer blow. 
The amount of silicon is very low for the acid process. 
Phosphorus remains practically constant at 0.1 per cent, 
and sulphur at 0.06 per cent. Figs. 9 and 10 are plotted 
on the same scale for comparison. The blow ends at 
9 minutes 10 seconds, and the rest of the curve results 
from the introduction of spiegeleisen. 

Open-hearth processes. — Steel is also made from 
pig iron in the hearth of a Siemens' regenerative furnace. 
(see Fig. 2). The silicon, manganese and carbon are 
removed by oxidation, as in the puddling, or in the Bes- 

* Plotted from experiments of F. Julian at the South Chicago 
works of Illinois Steel Company. See H. M. Howe, Journal Iron 
and Steel Institute, Vol. 11, 1890, page 102. 



METALLURGY OF IRON AND STEEL 



67 



semer, process. Two processes were formerly carried 
on in open-hearth furnaces: first, Siemens, or "pig and 
ore," process; second, Siemens-Martin, or "pig and 
scrap," process. These are combined into a single process 
in modern practice, and both scrap and ore are used. 

In the Siemens process pig iron is charged into the 
hearth and melted, part of the silicon and manganese 









TIME, MINUTES. 

2 3 15 6 


1 


5 9 
























S.3 
3.2 
3.1 
3.0 
2.9 
2.8 
2.7 
2.6 
2.5 
2.4 
2.3 
2.2 
2.1 
2.0 
1.9 
1.8 
1.7 
1.6 
1.5 
1.4 
1.3 
1.2 
1.1 
1.0 
0.9 
0.8 
0.7 
0.6 
0.5 
























































































































































































































!s 












































\ 










































s 






















































































\ 










































\ 










































\ 












































\ 










































\ 










































V 










































V 






















































































\ 




















































































\ 










































V 
































































































































\ 




























S L 














\ 




























sis 














\ 






























s 












\ 
































■^ 










































x 












. 


























N 




\ 












YZ 


























0.3 
0.2 
0.1 




\ 






\ 










t: 


























Mn N 
















_r 


























s 














^ 


i 




























" 






^^ 

































Fig. 10. 

being oxidized during the melting, and then rich ore is 
added to supply the oxygen to combine with the remain- 
ing silicon and manganese, and the carbon of the iron 
carbide. When the action is complete the bath of nearly 
pure iron is recarburized by the addition of spiegeleisen 
or ferromangariese, and the manganese reduces the fer- 
rous oxide present, and removes occluded gases or causes 
them to be dissolved, as in the Bessemer process. 

In the Siemens-Martin process pig iron is charged into 
the hearth, and melted with partial removal of the silicon 



68 MATERIALS OF MACHINES 

and manganese, and then steel scrap is charged into the 
bath, which melts, and thus the percentage of silicon, 
manganese and carbon is reduced by dilution. The 
remaining part of these substances is removed by the 
direct action of the oxidizing flame, and the indirect 
action of the ferrous oxide formed at the surface of the 
bath, and mixed with it. Spiegel or ferro are added 
as in the Siemens process. Ferrosilicon and ferroalumi- 
num are sometimes used in place of ferromanganese 
for the recarburization, the removal of iron oxide and the 
prevention of porosity. 

Either acid or basic lining may be used in the furnace 
in which the open-hearth processes are carried on. With 
the acid lining no phosphorus is removed and hence low 
phosphorus pig must be used. With the basic lining 
the phosphorus is removed as in the basic Bessemer 
process. 

Duplex process. — In modern practice in large steel 
plants the Bessemer converter and the open-hearth furnace 
are sometimes operated in combination. A charge is 
" blown" in an acid converter to remove silicon and 
manganese and a portion of the carbon; the charge is 
then transferred to a basic open hearth where the re- 
mainder of the carbon and the phosphorus are removed. 
This reduces the time in the open hearth and increases 
the output, and the converter may be lined with ganister, 
which is much cheaper and more durable than a basic 
lining. Another advantage is that the silica, formed by 
burning the silicon of the charge, is excluded from the 
basic furnace where phosphorus is removed, and hence 
does not need to be fluxed with lime; the result is economy 
of lime and increased durability of the open-hearth lining; 
this method also makes it possible to use a pig iron 
high both in silicon and phosphorus, which would be un- 



METALLURGY OF IRON AND STEEL 69 

desirable either in the basic converter or basic open hearth 
used separately, because of expense for lime and difficulty 
of slag disposal. 

Ductile castings. — Many machine members of some- 
what complicated form need to be strong and ductile. 
If such parts were required in large numbers, they could 
usually be produced by the process of casting, much 
cheaper than by the process of forging. For this reason 
much attention has been given to the production of 
ductile castings. The most important resulting processes 
are those for the production of malleable castings and 
steel castings. Some of the grades of brass and bronze 
give castings which are strong and ductile, but the high 
cost puts them out of competition for many purposes. 

Malleable castings. — Castings of the required form 
are made of iron that cools with all of its carbon in the 
combined state. These castings, which have a white 
fracture, and are hard, weak and brittle, are packed in 
cast-iron boxes surrounded with coarsely divided oxide 
of iron, usually hematite ore or hammer scale. These 
boxes are sealed and brought to a temperature of full 
redness, from 1500° to 1600° F., as quickly as possible in 
a reverberatory oven and are held at this temperature at 
least 60 hours, and are then cooled very slowly, the slow 
cooling being quite important. Cast forms are thus pro- 
duced that are very much like wrought iron in strength, 
ductility, resilience and softness. This process is called 
mallifying or annealing. Two changes occur to produce 
this result: first, the total carbon is reduced; second, 
the combined carbon is nearly all changed to graphitic 
carbon or " temper graphite," as it is called. 

First change. — The reduction of total carbon in the 
mallifying process occurs chiefly near the surface of the 
casting. It is probable that, at the temperature of 



70 MATERIALS OF MACHINES 

mallifying, carbon of the surface iron unites with oxygen 
of the iron oxide, or with oxygen of the imprisoned air, 
forming gaseous carbon monoxide or carbon dioxide which 
passes off; there is then a tendency to movement of 
carbon from the inner portions of the casting toward the 
surface where it in turn may Combine with oxygen and 
be removed.* The quantity of carbon thus removed 
must be a function of temperature, time of exposure to 
the decarbonizing conditions and of distance from the 
surface of the casting to the middle. Therefore, with a 
given time for annealing, thin castings might be quite 
transformed as to physical qualities, while thick castings 
might be only slightly changed. The removal of carbon 
by this means results in the formation of a surface layer 
which shows a wrought-iron-like fracture, while the rest 
of the fracture is black because of the second change. 
It is probable that migration of carbon occurs to only a 
slight extent in the ordinary American mallifying process. 
Second change. — When molten cast iron with high car- 
bon content cools under ordinary conditions, part of the 
combined carbon becomes graphitic carbon, appearing 
as flakes distributed throughout the cooled gray iron. 
The proportion of carbon thus becoming graphitic in- 
creases with the increase of total carbon, with the increase 
in time of cooling, with increase in silicon and decreases 
with increase in manganese; see page 127. But when 
iron contains less than 3 per cent of carbon with ordi- 
nary rate of cooling and with low silicon, the molten iron 
cools white, all carbon being in combination. If this 
white iron is raised to a temperature of from 1500° to 
1600° F. as in the mallifying process, the combined carbon 

* The fact of movement of carbon in solid iron at high tempera- 
tures is shown by the case-hardening process and by the cementation 
process; see pages 172 and 57. 



METALLURGY OF IRON AND STEEL 71 

tends to change into temper-graphite, and if the tempera- 
ture is maintained long enough the change may be quite 
complete. The graphite thus produced takes the form 
of very minute particles nearly uniformly distributed, 
which interrupt the continuity of the iron structure 
much less than the graphite flakes in gray iron; its pres- 
ence, therefore, has less effect to reduce strength and 
ductility than the graphite of the gray iron castings. 
During the change of combined carbon into temper 
graphite the resultant effect is to increase strength and 
ductility, because reduction of combined carbon has 
much greater influence in increasing these qualities than 
the presence of the temper graphite has to reduce them. 
The first change is probably more effective in increasing 
ductility, but it can only be satisfactorily accomplished 
in light castings. The second change occurs in heavier 
castings and increases ductility, though in less degree 
than the first change. This may be made clearer by an 
experiment;* a casting having one portion about f inch 
thick and another portion about T \ inch thick was put 
through the mallifying process. Tests of the mallified 
castings for carbon showed results as follows : 



Form of carbon 


Thick part 


Thin part 


Graphite 


Per cent 

2.93 
0.04 

2.97 


Per cent 

1.72 


Combined carbon 

Total carbon 


0.20 
1.92 







There was a much greater reduction of total carbon in the 
thin part, but a more complete conversion of combined 
carbon into temper graphite in the thick part. The thin 
part was much more ductile than the thick part. 

* This experiment was made at the works of the Westmoreland 
Malleable Iron Co. 



72 



MATERIALS OF MACHINES 



During the early years of development and use of this 
process the iron for the castings was white pig iron from 
cold-blast charcoal furnaces, because low carbon and low 
silicon were necessary for success in the mallifying process. 
This iron was melted in regular foundry cupolas with 
very little chemical change. But in present practice gray 
pig iron from coke furnaces is melted in a reverbera- 
tory furnace, called an "air furnace/' and the fluid iron 
is subjected to an oxidizing flame until silicon, manganese 
and carbon are reduced as low as is consistent with fluidity 
necessary for the production of " sharp " castings. 

The changes that occur in very light castings during 
the mallifying process are shown by the following average 
results of several analyses: 



Condition of castings 


Total carbon 


Graphitic 
carbon 


Combined 
carbon 


Before mallifying 

After mallifying 


Per cent 

2.79 
1.74 


Per cent 

0.177 
1.565 


Per cent 
2.613 
0.175* 







* Analyses made by Mr. W. H. McCord at the chemical laboratory of Stanford 
University. 

Reduction of total carbon in the castings for the malli- 
fying process is sometimes accomplished by charging 
steel or wrought-iron " scrap" into the cupola or air 
furnace; this reduces the carbon by dilution. 

Long experience has shown that the iron charged into 
the air furnace should contain about the following per- 
centages of the substances given: 

Per cent 

Silicon 1.25 

Manganese 0.40 

Phosphorus 0.15to0.2 

Sulphur, not over 0.05 

Carbon 3.5. 



METALLURGY OF IRON AND STEEL 73 

In the furnace the silicon is reduced to from 0.7 per cent 
to 1 per cent; manganese and sulphur are only slightly 
changed, while carbon is reduced to about 2.75 per cent. 
The phosphorus is not removed, but its presence in nearly 
the specified amount is desirable, because it renders the 
molten iron more fluid for casting and does not harm the 
product. 

If it is necessary to heat malleable castings for any 
purpose, as for straightening or bending, great ' care is 
necessary because, if the temperature is raised much 
above the mallifying temperature, from 1500° to 1600° F. 
the temper graphite combines again with the iron and the 
castings become brittle as they were before mallifying. 

Steel castings. — Some cast machine members need 
to be stronger and more ductile than malleable castings; 
other cast machine members are too thick to mallify 
satisfactorily. To meet such cases steel castings are made 
by 'pouring molten steel into molds directly from the 
steel-making process. When this is done the steel foun- 
dry includes steel making as well as steel founding. 

Converters and open-hearth furnaces are used in steel 
foundries though they are usually somewhat smaller than 
those producing steel ingots in steel mills. Special con- 
verters are used like the Tropenas or the Stoughton types.* 
In these converters all or part of the air enters above the 
surface of the metal bath and oxidation of the substances 
to be removed occurs by direct- combination with oxygen 
of the air and by indirect combination with the oxygen 
of the iron oxide, which is formed and mixed with the 
bath and reduced again throughout the blow. Obviously, 
the carbon is burned to carbon dioxide (see page 64) 
and the entire heat of combustion of the carbon is util- 

* See " The Metallurgy of Iron and Steel " by Bradley Stoughton; 
McGraw-Hill Book Co., page 272. 



74 MATERIALS OF MACHINES 

ized to help fix the temperature of the bath through the 
necessary range. 

Reheating and working of steel ingots by mill processes 
improves strength and ductility ; but steel castings do not 
get the benefit of this improving process, and hence, the 
pig iron used for making steel for castings must be lower 
in sulphur, and in phosphorus also if the acid process is 
used, than that used for ingots, in order that the castings 
may equal the forgings, made from the ingots, in strength 
and ductility. This means a more expensive pig iron, 
which adds to the cost of the steel castings. 

Basic steel is more apt to have porous spots and "blow 
holes " when cast than acid steel,* and these defects are 
especially undesirable in steel castings, because they cannot 
be welded up as in ingots. They may be a hidden source 
of weakness in stress members; and often much useless 
expense for labor is incurred when a "last cut " reveals 
porous defects that lead to the rejection of a nearly 
finished casting. The presence of manganese or silica 
tends to prevent porosity, and one or other of these sub- 
stances is introduced in the recarburization; but it is 
difficult to hold silicon from oxidization during "teem- 
ing " or casting, which occupies more time for steel cast- 
ings than for ingots, and if manganese is introduced in 
sufficient quantities to protect the iron from oxidation it 
may appear in the product in sufficient quantity to cause 
weakness and brittleness. 

To avoid porosity and brittleness, due to the presence 
of oxygen or iron oxide, a small amount of pure aluminum 
is sometimes introduced into the melt before pouring; 
the aluminum combines with the oxygen and is almost 
entirely removed in the slag. 

Steel for castings is sometimes obtained by melting 
* See page 63. 



METALLURGY OF IRON AND STEEL 75 

proper mixtures in crucibles. Obviously the fuel and 
labor cost is higher, but in the castings there is less lia- 
bility of formation of blow holes, and it is easier to control 
the content of substances other than iron than in the open- 
hearth or the converter. 

Steel that is cooled from a molten state has a rather 
coarse crystalline structure; this can be transformed into 
a fine structure with accompanying gain in strength and 
ductility by reheating and rolling or hammering at suit- 
able temperatures (see page 171) or by suitable heat 
treatment without mechanical working (see page 168). 
In case of steel castings the refining of structure must, 
of course, be accomplished without mechanical working. 

Steel for castings must be poured hotter than steel for 
ingots, since in the former case the metal must "run 
sharp," in order to take the required form in the mold, 
whereas this is of little importance in the case of ingots. 
Because of this, higher grade refractory material is needed 
for the linings in the steel foundry, and cost of furnace 
repairs is higher, than in the steel mill. 

The molds for steel castings are made both of " green 
sand" and of "dry sand"; but the sand in either case 
must be more refractory than that used in molds for 
iron castings, because the casting temperature of steel 
is probably from 2800° F. to 3000° F., while that for 
cast iron is about 2400° F. 

There is probably a difference of about 400° F. or 500° F. 
between the temperatures of solidification of steel and 
cast iron, and, since solid shrinkage begins when solidi- 
fication is complete, it follows that shrinkage is greater 
in steel, and, therefore, greater care must be taken both in 
the design of the steel castings and in the regulation of 
the rate of cooling of different parts of the casting that 
have different thickness of cross section. The shrink- 



76 MATERIALS OF MACHINES 

age stresses are removed if the casting is reheated for heat 
treatment to refine structure. See page 168. 

Steel castings, like steel ingots, are made with varying 
carbon content to meet the varying demand for strength, 
ductility and capacity for shock resistance. Increase in 
strength obtained by increase in carbon is accompanied 
by reduction of ductility; the converse also is true. 
Shock resistance increases both with strength and duc- 
tility, but the increase is greater with ductility than with 
strength, and therefore there is usually a resultant increase 
in shock resistance with reduction of carbon. 

Steel castings contain from 0.15 per cent to 0.7 per cent 
of carbon according to the required service. About 
0.1 per cent to 0.35 per cent of silicon is usually present, 
the lower value for soft castings and the higher value for 
hard castings. This amount of silicon does not diminish 
toughness of the metal itself and its presence seems to 
reduce the amount of iron oxide and free oxygen present 
with their undesirable effects. Sulphur and phosphorus 
should be kept below 0.05 per cent in castings that are 
to be subjected to severe stress. 



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CHAPTER V 

OUTLINE OF THE METALLURGY OF COPPER,* 
LEAD, TIN, ZINC AND ALUMINUM 

Copper is found in nature as native or pure copper, 
and in combination with many other substances. The 
only combinations that are of commercial importance 
are sulphides, oxides, carbonates and silicates. Whether 
the copper is pure or combined it is usually mixed with 
earthy "gangue," the amount of copper present varying 
from less than one per cent to 15 per cent or more. 

Copper is obtained from its ores either, (1) by "wet 
methods/' or hydrometallurgy, or (2) by "dry methods," 
or smelting. In (1) the copper is taken up by some solvent 
and leached out of the ore, the gangue remaining prac- 
tically unchanged; in (2) there is partial separation of the 
metallic combinations from the gangue by reason of 
specific gravity, (a) without application of heat; or (b) 
with the ore in a molten state. Only dry methods, 
which produce over 90 per cent of the copper of commerce, 
will be considered here. 

The copper that is produced by these methods is usually 
quite impure and must be refined either by the smelting 
refining or the electrolytic refining process, or by both. 

Native copper. — The ore of the Lake Superior dis- 
trict contains from 0.5 per cent to 4 per cent of copper 

* For fuller treatment of this subject see "Practice of Copper 
Smelting" by Peters and "The Metallurgy of the Common 
Metals " by Austin. 

77 



78 MATERIALS OF MACHINES 

rather finely divided and distributed throughout the 
earthy material of wide lodes. This ore is concentrated 
without heat until the copper becomes from 30 per cent 
to 90 per cent or more of the " mineral " as the concen- 
trate is called. Ore from the mine is crushed and passed 
through steam stamps and then through a series of "jigs," 
where it is shaken to help separation by gravity, and 
where a stream of water washes away the lighter portions 
from the upper surface, leaving the concentrated mineral. 

This mineral is smelted in a reverberatory furnace 
where a fusible slag is formed either by proper mixture 
of mineral with different gangue content, or by introduc- 
tion of a suitable flux. Again gravity carries the heavier 
metal down and the fluid slag is removed and the copper 
is cast into ingots. 

Oxides, carbonates and silicates of copper. These 
ores are found in the upper portions of mineral deposits, 
having been formed by the decomposition of copper sul- 
phides through the agency of air and water. The smelt- 
ing of such ores may be accomplished in a furnace of the 
blast-furnace type, the carbon dioxide of carbonates being 
driven off by heat, the oxide of copper being reduced by 
carbon monoxide from the fuel, and the silica of silicates 
being removed in the slag. The gangue enters the slag, 
which is made fusible by proper mixture of ores or by the 
introduction of a flux. Molten slag and molten copper 
are drawn off at proper intervals. The copper loss in the 
slag in this process is large, because copper oxide combines 
with the silica, and because some metallic copper is carried 
away mechanically. This process is not very extensively 
used now because of growing scarcity of oxide ores, and 
also because it is advantageous to charge oxide ores with 
sulphide ores to help supply the necessary oxygen for the 
removal of sulphur as sulphur dioxide. 



METALLURGY OF COPPER, ETC. 79 

Copper sulphide ores. — The sulphide ores of copper 
contain not only copper sulphide but also iron sulphide 
together with the gangue. If such ore is melted in a 
neutral or reducing atmosphere the copper and iron sul- 
phide will melt into a "matte " which settles out of the 
ore by gravity, leaving the gangue which, if properly 
fluxed, may be drawn off. No sulphur is removed by 
this treatment; but if the ore is first roasted in an oxidiz- 
ing atmosphere at a temperature too low to melt the 
sulphides, part of the sulphur will burn and pass off as 
the gas S0 2 . Copper holds sulphur more strongly than 
iron does, and hence the iron sulphide yields its sulphur 
first and the iron deprived of its sulphur takes up oxygen. 
The product of carefully regulated roasting contains 
copper sulphide, iron oxide and silicious gangue; probably 
some copper oxide has been formed and some iron sulphide 
remains. When this product is smelted the copper and 
iron sulphides melt, carrying down any precious metals 
that may be present; the iron oxide acts as a flux for the 
silicious gangue, thus forming a fusible, removable slag 
while any iron sulphide present enters the matte which 
is drawn off and cooled as "coarse metal." In this proc- 
ess silica sometimes has to be added as a flux when the 
ore carries excess of iron and the gangue is deficient in 
silica. The roasting and smelting are repeated until all 
iron and gangue are removed and only copper sulphide, 
or "fine metal," remains. This fine metal is cooled in 
proper form and size and is roasted in an oxidizing atmos- 
phere; a part of the sulphur burns, while the copper thus 
freed from sulphur takes up oxygen, forming copper oxide. 
When this roasting has continued long enough to produce 
the right proportions of copper oxide and copper sulphide, 
the charge is melted and copper oxide gives up its oxygen 
to combine with the sulphur of the copper sulphide, pro- 



80 MATERIALS OF MACHINES 

ducing sulphur dioxide which passes off as a gas, leaving 
metallic copper. 

Blister copper, or black copper, the product of the 
three processes described, still contains substances that 
reduce value for industrial purposes; sulphur, iron and 
copper oxide are present; and there often are precious 
metals, gold, silver or platinum, that may be removed 
with profit; and often antimony, arsenic, bismuth, 
selenium and tellurium. These five substances, if present 
in the ore, are not entirely removed by the roasting or 
smelting processes. There are two refining processes: 
furnace refining and electrolytic refining. If it is found 
from analysis that precious metals are present in amounts 
that will make their recovery profitable, the furnace 
refining may be omitted and the copper cast into anode 
plates which are transferred to the electrolytic refinery: 
here they are suspended in an electrolyte consisting of a 
solution of copper sulphate in dilute sulphuric acid. A 
suitable current is maintained through the solution and 
the copper is transferred, through the agency of the 
electric energy, from the anode to the cathode, where it 
is deposited free from impurities. The precious metals, 
together with the other substances carried by the anode 
plates, fall to the bottom of the tank containing the elec- 
trolyte, forming " slimes " which are treated for the recov- 
ery of the precious metals, while the pure copper from the 
cathodes is prepared for the market. 

But if there is not a profitable content of precious 
metals the refining may be done by the furnace method. 
The blister copper is melted down in a reverberatory fur- 
nace, a slag forming during the melting, which is probably 
composed of silica present, and iron oxide formed during 
the melting; this slag is skimmed off and compressed air 
is forced through the bath of molten metal; the result is 



METALLURGY OF COPPER, ETC. 81 

that sulphur, arsenic, iron and copper are oxidized; the 
oxidized sulphur and arsenic pass off as gas, and the iron 
oxide combines with the remaining silica to form addi- 
tional fusible slag, which is removed. Some of the cop- 
per oxide formed acts as a carrier of oxygen, yielding its 
oxygen to combine with sulphur of any copper sulphide 
that remains; but a considerable amount of copper oxide 
is left in the bath after the removal of impurities is com- 
plete. The oxidizing atmosphere is now changed to a 
reducing atmosphere and green poles, branches of trees, 
are inserted so that the ends dip beneath the surface of 
the bath of molten copper; the evolution of steam and 
gas agitates the bath and the carbon of the "charred poles 
combines with the oxygen of the oxide of copper and forms 
carbon monoxide, leaving the pure copper, which is cast 
into ingots. 

Lead. — Many combinations of lead with other sub- 
stances occur in nature, but almost the entire supply of 
lead of commerce is obtained from the ore galena which 
consists of lead sulphide mixed with varying proportions 
of gangue. Sometimes the galena also bears silver and, 
more rarely, gold, while copper, antimony and arsenic 
are commonly present. If the ore does not contain paying 
proportions of silver or gold, it may be concentrated 
without heat by stamping and washing before melting. 
But if these precious metals are to be extracted no pre- 
liminary concentration is allowable because of loss of the 
precious metals in the gangue removed. 

Treatment of Galena. Roasting. — Crushed ore is 
charged into a reverberatory furnace and a low tempera- 
ture is maintained in an oxidizing atmosphere. The 
sulphur of a part of the lead sulphide, PbS, is oxidized 
and passes off as sulphur dioxide, S0 2 , while the lead that 
is left takes up oxygen, forming lead oxide, PbO; also 



82 MATERIALS OF MACHINES 

some lead sulphate, PbS04, is formed by oxidation of lead 
sulphide. These changes may be represented thus: 

PbS + 30 = PbO + S0 2 . 
PbS + 40 = PbS0 4 . 

Smelting. — After the roasting has continued long 
enough to produce proper proportions, the lead oxide and 
sulphate are mixed with the remaining lead sulphide, 
the temperature is raised in a neutral atmosphere and the 
following reactions take place: 

PbS + 2PbO = 3Pb + S0 2 . 
PbS + PbS0 4 = 2 Pb + 2 S0 2 . 

The metallic lead melts and settles in a low part of the 
furnace, whence it is cast into ingots. The remainder of 
the charge still contains a large amount of lead, and the 
roasting and smelting are repeated several times; ulti- 
mately no lead sulphide is left to reduce the remaining 
lead oxide and sulphate. Then fine carbon (coal) is intro- 
duced with the charge to take away oxygen in gaseous 
C0 2 from the lead oxide, and to reduce lead sulphate to 
sulphide, thus rendering the sulphur also available as a 
reacting agent. Thus an additional amount of lead is 
freed, melted and cast into ingots. During the smelting 
lime is added to render the charge more resistant to melt- 
ing, since it is desirable to melt the lead out from the solid 
residue. In this residue there still remains a considerable 
amount of lead, probably as oxide or sulphate, which 
cannot be removed by repetition of roasting and smelting; 
the amount may run as high as 30 per cent. Transfer 
may then be made to a blast-furnace for lead smelting 
where most of the lead may be recovered. Each of the 
successive smelting processes has a higher temperature 
than the preceding one and produces less pure lead. 



METALLURGY OF COPPER, ETC. 83 

This process is not suited to ores containing more than 
5 per cent of silica, since silica unites with lead oxide, caus- 
ing loss of lead in the slag. Other forms of furnace are 
used for smelting lead, but the chemical changes are the 
same as those given, the difference being in the details of 
operation. 

Ores containing lead, silver and gold, and usually copper, 
arsenic and antimony are smelted in a blast-furnace, an 
important function of the lead being to collect and carry 
down the precious metals into the "base bullion." Oxide 
ores of lead may be used directly, but sulphide ores are 
roasted so that the resulting oxide may be reduced by 
CO of the blast-furnace; some sulphide remains, however, 
in the roasted ore. The flux used is iron ore and lime- 
stone, and the ferric oxide, reduced to ferrous oxide, also 
acts with carbon to reduce the lead sulphide, as follows: 

PbS + FeO. + C = Pb + FeS + CO 

and also to reduce the lead sulphate as follows: 

PbS0 4 + FeO + 5 C = Pb + FeS + 5 CO. 

The molten lead not only carries down gold and silver, but 
also copper, arsenic and antimony. The remaining FeO 
unites with lime of the flux and the silica of the gangue to 
form a fusible slag. 

Softening. — The base bullion from the blast-furnace 
is treated first in a shallow reverberatory furnace, where it 
is melted and subjected, in an oxidizing atmosphere, to 
heat at a temperature that just melts the lead. At the 
surface lead, copper, antimony and arsenic oxidize and 
combine into a "dross" which floats upon the molten 
lead. This process is continued until tests show com- 
plete removal of the copper, arsenic and antimony. 
Then the charge is cooled just enough to harden the dross 



84 MATERIALS OF MACHINES 

which is skimmed off; the lead now contains only silver 
and gold, and is treated by the Pattinson process. 

The Pattinson process depends upon the fact that 
when a molten mixture of lead and silver is cooled slowly 
with constant stirring, crystals of lead very low in silver 
will form, and removal of these crystals leaves lead high 
in silver. This process is carried out in a series of cast- 
iron pots set in masonry with properly arranged furnaces 
for heating. Lead containing silver is melted in the 
first pot and then the source of heat is withdrawn; as 
the stirred metal cools the crystals of lead very low in 
silver are dipped into the second pot by means of a skim- 
mer that allows the molten metal richer in silver to run 
back into the first pot. Extension of the process eventu- 
ally gives lead very high in silver in the first pot and lead 
very low in silver in the last pot. The latter goes to the 
lead market, while the former is treated in the cupellation 
furnace, where the lead is all oxidized into PbO, litharge, 
leaving purified silver. Then the PbO is reduced by car- 
bon monoxide in a reverberatory furnace, leaving pure 
lead. 

The Pattinson process leaves copper, arsenic and anti- 
mony with the silver and hence produces a very pure, 
soft lead. 

Tin. — Nearly all the tin of commerce is extracted 
from the ore called " tinstone" or "cassiterite," which 
consists of a stannic oxide, Sn0 2 , mixed with gangue con- 
taining earthy and metallic substances. 

For removal of the earthy portion of the gangue the 
ore is stamped fine and washed on racks, where a stream of 
water carries away the lighter material from the surface, 
leaving the tin oxide and other heavy material. This 
concentrated ore, which usually contains iron and copper 
pyrites, FeS 2 and CuFeS 2 , and arsenical pyrites, FeSAs, is 



METALLURGY OF COPPER, ETC. 85 

roasted in an oxidizing atmosphere in a reverberatory 
furnace. Sulphur is removed as gaseous S0 2 ; arsenic 
forms the oxide, As 2 3 , the white arsenic of commerce, 
which is caught in long flues; while the copper becomes 
copper oxide and copper sulphate, and the iron becomes 
iron oxide. The soluble copper sulphate is removed from 
the roasted ore by washing. The washed ore, which still 
contains copper, iron, arsenic and sulphur, is then reduced 
by smelting with carbon in small shaft furnaces or in rever- 
beratory furnaces. In either case oxygen of the tin oxide 
combines to form carbon dioxide either with carbon mixed 
directly with the ore, or with carbon monoxide from 
partial combustion of coal used as a source of heat. A 
small amount of lime is used as a flux to remove silica 
that may remain. The ingots of crude or raw tin from 
the smelting process still carry not only copper, iron, 
arsenic and sulphur but often also lead, antimony and 
tungsten, which must be removed by refining. This refin- 
ing usually consists of two processes, liquation and boiling. 
The ingots of crude tin are piled on the hearth of a rever- 
beratory furnace and the temperature is slowly increased 
to a point where the tin melts out leaving the unfused 
impurities. The molten tin, still with small amounts of 
iron, arsenic and sulphur, is led into a receptacle, which 
has a separate source of heat, where green twigs in bundles, 
or wet sticks, are held submerged in the bath of tin. Evo- 
lution of steam and gas causes brisk agitation of the tin, 
whereby all parts are brought in contact with the air and 
the impurities are oxidized. A scum of a portion of the 
oxides thus formed collects on the surface. After the 
boiling, the still molten metal is allowed to stand for about 
an hour, when the scum is removed and the tin is ladled 
out into ingot molds. A part of the oxidized impurities 
goes out with the scum, but another part which is of higher 



86 MATERIALS OF MACHINES 

specific gravity than the tin settles toward the bottom. 
Thus, when the tin is ladled out, that which conies from 
the top, called " refined tin," is purer than the so-called 
"common tin" from the lower portions of the mass. 
The material at the bottom is often cooled and liquated 
and boiled again. 

The residue in the liquation furnace with increased 
temperature yields more tin of lower grade. 

Sometimes a process called " tossing " is substituted 
for boiling; the molten tin is dipped in ladles and allowed 
to run back into the bath from a considerable height. 

There are effective methods for recovering a large por- 
tion of the tin which remains in the smelter slags and in 
refining dross. 

Zinc. — The ores from which zinc of commerce is 
extracted are: 

Zinc blende, which consists of zinc sulphide, ZnS, 
mixed with earthy gangue and usually with manganese, 
iron, cadmium and silver; more rarely it bears mercury, 
gold, lead and tin. 

Calamine, made up of zinc carbonate, ZnC0 3 , with 
cadmium, iron and manganese as carbonates, and with 
lead sulphide and iron oxide, and earthy gangue. Zinc 
silicate, Zn 2 Si04, is also sometimes present. 

Franklinite, which is made up of oxides of iron, man- 
ganese and zinc with earthy gangue. 

Zinc vaporizes at a temperature of about 1725° F. and 
advantage is taken of this fact in smelting. Zinc oxide 
and coal, both finely divided, are mixed and heated in 
muffles or retorts, where the zinc oxide is reduced by the 
carbon with formation of carbon monoxide, and where 
the metallic zinc is vaporized, led away and condensed. 

Since the ore for this purpose must be in the form of 
zinc oxide, it is necessary to roast the ores containing 



METALLURGY OF COPPER, ETC. 



87 



zinc sulphide — zinc blende — in an oxidizing atmos- 
phere to oxidize and remove the sulphur as SO2 and to 
oxidize the zinc. 

Zinc carbonate ore — calamine — is usually roasted to 
remove moisture and to drive off carbon dioxide from the 
carbonate in order to produce the zinc oxide for smelting. 

The Belgian process for smelting zinc is chiefly used 
in the United States. Retorts, which are refractory 
cylinders about 8| inches in diameter and four feet long 
(see Fig. 11) are set in tiers in a chamber having a 
source of heat. The inner end of the retort is closed, and 
a refractory cone C is inserted in the outer open end. 
The retorts are charged 
with the fine mixture of 
coal and zinc ore, usu- 
ally moistened so as to 
cohere, and the heat from 
the furnace raises the 
temperature of the 
charge. Water is driven 
off as steam which passes 
out through the cone. 
The carbon of the coal 
unites with the oxygen of 
the zinc oxide forming 
carbon monoxide which 
passes through the outer opening of the cone where it 
burns — with a blue flame — to carbon dioxide. The zinc 
thus isolated is vaporized and the vapor passes to the cone 
where, on meeting the cooler walls, it is condensed and the 
liquid zinc is withdrawn at proper intervals. When the 
charge is exhausted the cone is removed, the residue is 
withdrawn, the retort is recharged, the cone is replaced 
and the process begins again. 




88 MATERIALS OF MACHINES 

Aluminum. — Aluminum occurs very abundantly in 
nature, but it is always in combination with other sub- 
stances, such as oxygen, sodium, fluorine and silicon. 

In its combinations with silicon and oxygen, aluminum 
is useful for refractories and in the ceramic arts. See 
Chapter III. 

Metallic aluminum may be produced by several methods, 
but the most important process commercially is elec- 
trolysis, or decomposition by an electric current, of alu- 
mina, A1 2 3 . Pure alumina is very infusible; and, since 
a substance must be fluid for electrolysis, it was necessary 
to find a solvent for alumina that would melt at a rela- 
tively low temperature, and that would allow the decom- 
position of the alumina without being affected itself. 
In 1889 a patent was granted to Charles M. Hall, covering 
the use of cryolite, a fluoride of aluminum and sodium, 
3 NaF, AIF3, as a solvent bath for electrolysis of alumina. 
This substance melts at a red heat, and when melted 
dissolves alumina, thus forming an electrolyte which can 
be decomposed by a suitable electric current. 

The process is carried out in rectangular cast-iron pots 
having a lining of hard-baked carbon about 3 inches 
thick which forms the negative electrode or cathode. 
The positive electrodes or anodes consist of cylindrical 
carbons about 3 inches in diameter and originally about 
15 inches long; these are suspended with axes vertical 
by f-inch rods of copper, which in turn are clamped to a 
copper bar that extends above the pot throughout its 
entire length. This bar and the carbon lining of the pot 
are connected into a current to which electrical energy 
is supplied at low voltage. The anodes are lowered until 
they touch the cathode which is the lining of the pot. 
This completes the circuit and electricity flows. The 
anodes are then withdrawn slightly to form an air gap, 



METALLURGY OF COPPER, ETC. 89 

thus providing an electric furnace of the arc type. Cryo- 
lite for the solvent bath is then introduced and is melted 
by the heat from the electrical energy supplied. Pure 
alumina is then stirred into the bath and electrolysis 
begins. The A1 2 3 is decomposed by the action of the 
current, aluminum being deposited on the cathode or 
pot lining, while oxygen appears at the surface of the 
anodes, combines with the anode carbon and passes off 
as gaseous carbon dioxide, thus causing the anodes to 
waste away and to require periodical renewal. The me- 
tallic aluminum is dipped or syphoned out at intervals 
and cast into ingots. 

The alumina for this process must be free from other 
substances; and, since pure alumina is not found in nature, 
a purification process is necessary. The source of the 
alumina is usually bauxite, which has already been de- 
scribed (see page 34) as a " mixture of a large proportion 
of hydrated alumina, A1 2 3 • 2 H 2 0, with clay, silica, iron 
oxide and titanic oxide and often with another hydrated 
aluminum oxide, A1 2 3 • 3 H 2 0." The separation of the 
alumina from the other substances of the bauxite is 
effected as follows: 

The bauxite is ground fine and mixed with fine sodium 
carbonate; the mixture is stirred and heated in a furnace 
and the alumina displaces the carbon dioxide of the sodium 
carbonate, driving off the gaseous carbon dioxide and 
forming aluminate of soda. This may be expressed chem- 
ically as follows : 

Al 2 3 -3 H 2 0+3 Na 2 C0 3 = Al 2 3 .3 Na 2 0+3 C0 2 +3H 2 0. 

The aluminate of soda thus formed is soluble in water, 
while the other substances in the bauxite remain un- 
changed and are insoluble. When the reaction is shown 
by chemical tests to be complete the charge is withdrawn 



90 MATERIALS OF MACHINES 

and cooled and the aluminate of soda is taken into solu- 
tion in warm water and thus separated from the unde- 
sirable substances. Then carbon dioxide is forced through 
the solution of aluminate of soda and soluble sodium 
carbonate is formed, and the alumina which is precipi- 
tated is filtered out, washed and dried and is ready for 
electrolysis. The sodium carbonate is recovered to use 
again in the process. 

Alumina is also obtained by making an intimate mix- 
ture [of very fine cryolite with calcium carbonate (chalk) 
and roasting the mixture; the chemical reaction is as 
follows : 

2 (A1F 3 - 3 NaF) + 6 CaC0 3 = A1 2 3 • 3 Na 2 

+ 6 CaF 2 + 6 C0 2 . 

The products are gaseous carbon dioxide, insoluble calcium 
fluoride and aluminate of soda, which may be dissolved 
and treated as in the last process to produce pure alumina. 



PART SECOND — PHYSICAL PROPER- 
TIES OF MATERIALS 



CHAPTER VI 
TESTING MATERIALS 

Machine members in service are subjected to the 
action of external forces which tend to break or distort 
them. Some members, like springs, fulfill their function 
by yielding periodically through considerable space to 
applied forces; but a large proportion of members require 
rigidity, that is, the yielding under applied forces must 
be kept very small. In any case permanent distortion 
and breakage must be prevented. 

The designer of machines must know the effect of 
external forces applied to the materials of machines. 
This knowledge is derived from tests. A test piece of 
suitable dimensions may be made of any material and a 
steadily increasing force may be applied to it until it 
breaks or is very much deformed. The force applied to 
the test piece may tend to crush it, a compressive force, 
or to pull it apart, a tensile force, or to bend it, a trans- 
verse force, or to twist it, a torsional force, or the external 
forces may produce some combination of these tendencies. 

A solid resists change of form; forces applied to a solid 
tending to change its form induce stress within it; 
stress may be defined as the action and reaction between 
adjacent parts of a solid during resistance to change of 
form. 

91 



92 



MATERIALS OF MACHINES 



First illustration. In Fig. 12, suppose the tensile force P 
is applied to a cylindrical test piece; in any cross section 
like AA there will result an action and reaction between 
adjacent faces of the section resisting separation; this 
action and reaction is called tensile stress. If P equals 



Fixed 
End 



■^P 



A 

Fig. 12. 

5000 pounds the total stress in the section AA is 5000 
pounds; if the cross-sectional area of A A equals one 
square inch the unit stress in the section is 5000 pounds 
per square inch. If the area of cross section were \ square 
inch, the unit stress would equal 5000 -s- \ = 10,000 pounds 
per square inch. Reversal of the direction of the external 
force P would change it from a tensile force to a compres- 
sive force tending to crush the test piece, and in all sec- 
tions like AA a compressive stress would result. 





x 1 


1 


\ 


P 


1 x 


w% 


•ill 
1 




L 




pur 

f 



Fig. 13. 



Second illustration. In Fig. 13, suppose a transverse 
force P applied tending to bend a test piece of rectangular 
cross section. If the test piece is bent by this force, the 
fibers below a neutral axis XX would be stretched, while 



TESTING MATERIALS 



93 



those above would be shortened; hence the bending force 
would produce in any section both tensile and compressive 
stress. Also the force P tends to cause the adjacent 
surfaces in any section like A A to slide over each other and 
thus there is produced shearing stress in the section. 

Third illustration. The test piece may be subjected to a 
force that tends to twist it about its axis, as in Fig. 14. 
Then in any section AA there is a tendency for one surface 
to slide over the adjacent surface about the axis of the 
test piece, and hence shearing stress is induced. 

Testing machines have been designed and constructed 



Eixed 
End 



Applied at right angles to paper 
P 

A 




End View 



Fig. 14. 



capable of applying definite forces to test pieces in these 
several ways,* with devices for measuring the defor- 
mation corresponding to any applied force. In whatever 
way the force is applied to the test piece the object of 
the test is to record simultaneous values of stress and 
deformation, because a knowledge of the relation of these 
values enables the designer to proportion machine parts 
for safety from breakage, for necessary rigidity in opera- 
tion, and for economy of material. 

A tension test of ductile material will now be consid- 
ered for illustration. The increasing tensile force P is 

* See " Experimental Engineering," by Carpenter and Diederichs, 
Chapter IV. 



94 MATERIALS OF MACHINES 

applied as in Fig. 12 and the resulting change of form — 
deformation — is elongation, accompanied by correspond- 
ing reduction of section area. Assume that P is applied 
in successive increments; there will be corresponding 
values of p, representing unit stress in any cross section, 
and, if the original section area is represented by F, 
the successive values of p will equal the corresponding 
values of P -s- F* 

Assume also that after each increment of stress an 
accurate measurement of elongation is made. In the 
early part of the test the elongation is proportional to 
stress; but after passing a certain limit, called the elas- 
tic limit, the elongation becomes increasingly greater for 
a given increase in stress. The law may be stated thus: 
deformation is proportional to stress within the elastic 
limit. 

If, before reaching the elastic limit, the stress is grad- 
ually reduced to zero, the elongation becomes zero also; 
that is, the test piece returns to its original dimensions. 
Within this limit the material may be considered per- 
fectly elastic, f since elasticity may be denned as the 
property whereby a material returns to its original 
dimensions on relief of stress. If, however, the elastic 
limit is passed before relief of stress, the test piece will 
be permanently elongated. This permanent elongation 

* Since F decreases with increase of P the successive values of p 
strictly should equal the corresponding values of P divided by the 
corresponding values of varying F. In practice, however, F is con- 
sidered constant and equal to the original section area. 

f It is probable that even within the elastic limit the material 
is not perfectly elastic. Very refined measurements show that 
materials take some " set" even under relatively small stress. The 
values of this set, however, are so very small that they may be 
safely disregarded in the ordinary testing of the materials of en- 
gineering. 



TESTING MATERIALS 95 

is called " set." The elongation that disappears on relief 
of stress is called elastic deformation. 

After passing the elastic limit the elongation accom- 
panying a given increment of stress increases steadily 
until finally the maximum stress that the test piece is 
capable of sustaining is reached and rupture occurs. 
This maximum stress is a measure of the ultimate strength 
of the material. 

If the test piece were divided into equal units of length, 
say one inch, as in Fig. 15, and if the material were 
absolutely homogeneous and of equal strength in all 
sections, it would follow that with increasing stress all 
units of length would share equally in the elongation 



*— End for fastening — > 


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Fig. 15. 

and, on reaching a stress corresponding to the ultimate 
strength of the material, all sections would yield at once. 
But no such material is available for machine parts and 
when the ultimate strength of the weakest section is 
reached, local yielding occurs, a "neck " forms and the 
piece breaks. 

Fig. 16 shows a tension test piece before and after test- 
ing. The original piece is subdivided equally by punch 
marks into half -inch units of length. The tested piece 
shows the increase in length of these units, and also the 
local reduction of area and fracture at minimum section. 

As the test goes on the equal divisions, see Fig. 15, 
share the elongation almost equally until maximum stress 
is reached. At any point in the test there is a total 
elongation represented by X expressed in inches and there 



TESTING MATERIALS 97 

is a corresponding unit elongation (or elongation of each 
of the one-inch divisions) represented by e, and expressed 
in inches per inch of original length I, of the tested section. 
It follows then that 

X 

After passing the maximum stress the elongation and 
reduction of section area are localized at or near the 
neck. 

The simultaneous values of unit stress p and of relative 
elongation e may be plotted with reference to rectangular 
axes (p values being laid off vertically upward and e 
values being laid off horizontally toward the right) and 
through the points thus located a curve may be drawn 
called the stress-deformation diagram. In what fol- 
lows, this name will be abbreviated to s.-d. diagram. 

Fig. 17 shows s.-d. diagrams for mild steel. Diagram I 
is plotted on such a scale for X values that the entire 
diagram up to breaking falls within the limits of the 
figure. The yielding, however, on this scale is too small to 
show until a unit stress of nearly 33,000 pounds is reached. 
Hence diagram II is plotted with greatly increased scale 
for values of e, and this diagram shows the test only as 
far as M , diagram I. This diagram is useful as showing 
more clearly what occurs in the early part of the test. 

Starting from A with stress and deformation equal to 
zero the line of the diagram is straight until Bi is reached. 
This point Bi is the elastic limit which is strictly defined 
as the point where proportionality of stress and defor- 
mation ceases; or, otherwise expressed, it is the point 
where the diagram line ceases to be a straight line. Usu- 
ally in ductile materials the diagram line then curves to 
the right until the yield point is reached, where it becomes 



98 



MATERIALS OF MACHINES 









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TESTING MATERIALS 99 

about horizontal, indicating a considerable yielding 
without increase of stress.* At C\ further elongation 
requires increase of stress and the diagram line rises in a 
curve to M. In engineering testing the yield point is 
usually taken at the elastic limit. 

If on reaching some point within the elastic limit, N, for 
example, stress had been reduced gradually to zero, the 
point tracing the diagram line would have retraced the 
line to A, and the test piece would have recovered its 
original dimensions. 

But if the relief of stress had been delayed until some 
point beyond the elastic limit R was reached, the tracing 
point would return to the X axis over the line R T nearly 
parallel to BiA. During this return a certain portion of 
the elongation (elastic deformation) ST would disappear, 
while another portion, TA, would remain as permanent set. 

The physical properties that appear on the s.-d. diagram 
are: 

1. Strength at elastic limit. 

2. Strength, ultimate. 

3. Ductility. 

4. Elasticity. 

5. Stiffness. 

6. Resilience, elastic. 

7. Resilience, ultimate. 

1. The strength at elastic limit is proportional to the 
ordinate A B, whose value may be read in pounds per 
square inch. 

2. The ultimate strength is proportional to the maxi- 
mum ordinate DDi, whose value in pounds per square 
inch may also be read. 

* For an explanation of the probable reason for the horizontal 
part of the diagram line, see page 155. 



100 MATERIALS OF MACHINES 

3. A material is ductile that stretches under an increas- 
ing tensile force before it breaks. Ductility is therefore 
proportional to the amount of stretching. Hence it is 
proportional to the length of the s.-d. diagram on the axis 
of X. If s.-d. diagrams of different materials are plotted 
with the same scale for values of e, the relation of their 
ductilities can be found by comparison of the lengths of 
the diagrams on the X axis. The value of e at rupture 
might be taken as a measure of ductility; but in engineer- 
ing test practice it is customary to measure the elonga- 
tion of the tested section of the test piece after rupture, 
and to compare this with the original length of the tested 
section. Ductility is thus expressed as per cent elonga- 
tion in I inches, I being original length of tested section. 
It is necessary to specify the original length because, 
from the maximum stress until rupture, the elongation is 
localized and is not shared by all one-inch sections alike, 
hence the average elongation depends on how many of 
the unnecked sections are included with the necked section 
in finding the average. For example, the average elonga- 
tion derived from the necked section taken with two other 
sections would be greater than the value derived from the 
necked section with any greater number of other sections.* 

4. Elasticity. — When the initial part of the s.-d. dia- 
gram is a straight line, it is an indication that the material 
is practically perfectly elastic for the corresponding 
range. Thus in diagram II, Fig. 17, if the material is 
perfectly elastic for the range corresponding to AB, it 
could be stressed within these limits an indefinite number 

* The value of e for the point E in diagram I is derived from 
the permanent elongation after rupture; whereas for all previous 
points of the diagram the elastic elongation is included with the 
permanent elongation for the computation. The error due to this, 
however, is small enough to be negligible. 



TESTING MATERIALS 



101 



of times and each time it would return to its original di- 
mensions. But in an s.-d. diagram like ACB, Fig. 18 — 
which is a record of a test of cast iron — there is no elastic 

































































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range and even small stress produces set. In this test 
stress was relieved at C and the diagram line traced the 
path CDE, showing a set at E * proportional to EG. Then 

* Stress was not reduced to zero because of difficulties that 
would result in manipulation of the apparatus for measuring elon- 
gation. 



102 MATERIALS OF MACHINES 

stress was again increased until rupture occurred and the 
diagram line followed the path EFCB. It is evident that 
upon relief and reapplication of stress the line is not 
straight from C to E and again from E to C. The disap- 
pearance of elastic deformation occurs at practically a 
uniform rate from C to D — or CD is a straight line — 
and from some point D the rate of disappearance of elastic 
deformation increases to E; that is, DE is curved. The 
line of reapplication of stress EEC similarly is straight 
from E to some point F, while FC is a curve ending at 
the starting point C* 

It is obvious that the first application of stress has 
produced an artificial elastic limit F, together with a cor- 
responding elastic range EF, and the stressed material 
may be considered as artificially elastic through this 
range. 

The width of the loop CDEFC is only slightly affected 
by the time occupied in changing stress from C to E and 
back again to C; and hence the loop phenomenon must be 
a function of some quality of the material. Very careful 
measurements lead to the conclusion that the same phe- 
nomenon occurs with ductile material, but the corre- 
sponding values are too small to be detected by the 
measuring instruments of ordinary testing practice of 
engineering. 

5. Stiffness of a material is a function of the amount of 
yielding under given force. It is measured by the ratio 
within the elastic limit of unit stress to relative elonga- 
tion = -, and this ratio is called the modulus of elas- 



* If relief of stress had been complete the curve DE would have 
continued to the X axis and would have returned at the left of EF, 
but a final curve would have brought the line to the same destination 
C as in starting from E. Thus the loop would be made wider. 



TESTING MATERIALS 103 

ticity for tension, E t * Since for the material represented 
by Fig. 17 p is proportional to e within the elastic limit, it 
follows that any corresponding values of p and e between 
A and B — those at N, for example — may be taken to 
compute the value of E t . The variation in stiffness has 
much narrower limits than tensile strength; thus, low- 
carbon open-hearth steel may have a tensile strength of 
about 60,000 pounds per square inch, while high-carbon 
crucible steel may have a tensile strength above 100,000 
pounds per square inch, a variation of 100 per cent, proba- 
bly; yet the values of E t corresponding may be 28,000,000 
and 32,000,000, a maximum increase of about 14 per cent. 
Practically all values of E t for steel — of whatever grade — 
fall within the limits just given. 

In the tension s.-d. diagram for cast iron, Fig. 18, ABC, 
the stress-deformation line is curved continuously from 

v 
start to rupture, and hence the value of E t = - varies 

continuously and the original material cannot be said to 
have a definite modulus of elasticity. But since the 
material by relief and reapplication of stress from some 
point — as for example, C — is given an artificial elastic 
range, it follows that the stressed material would have an 
artificial value of E t , measuring stiffness, corresponding to 

- for any point in the straight line EF. This value of 

artificial modulus of elasticity, E t , for cast iron probably 
averages about 15,000,000 pounds per square inch. 

* The modulus of elasticity E t is qualitatively the same as unit 
stress p; that is, it is a value expressed in pounds per square inch; 

V I . . I . 

for, E t = — = p • - in which -, being a ratio of linear dimensions, 
e X X 

is an abstract quantity; hence E t equal to pounds per square inch 

multiplied by an abstract number must also equal some value 

expressed in the same units as p. 



104 MATERIALS OF MACHINES 

Resilience is the name given to the work done within 
a material while its form is changed by external forces. 
It is therefore the summation of the product of all 
stresses produced, multiplied by their yielding. But 
this summation is equal to the work done by external 
forces in producing change of form; hence this work 
is a measure of resilience. 

In s.-d. diagram II, Fig. 17, work is done by the unit 
force which is initially zero and which increases uniformly 
to the elastic limit. Let unit force at the elastic limit be 
represented by pi; then the average unit force up to the 

elastic limit equals -^ . This unit force acting upon one 

square inch section area has caused an elongation equal 
to € in each one-inch section of length of the test piece, 

and hence ^ e equals the work done on one cubic inch of 

the test piece in bringing it to the elastic limit. This 
value is sometimes called the modulus of resilience of the 
material and is represented by U t . 

The total elastic resilience, equal to the work done 
on the tested portion of the test piece up to the elastic 

limit, is equal to the total force P = ^ F, multiplied by 

the total elongation = X = d or is equal to PX = ^ eFl. 

This value is proportional to the area of the triangle AB X G 
and is equal to the modulus of resilience multiplied by 
Fl, the volume of the tested portion of the test piece. 

The ultimate resilience is the work done in breaking 
the test piece. Consider s.-d. diagram I, Fig. 17. The 
mean height of the diagram Y m measures the mean unit 
force acting throughout the test; while AEi — the final 
value of e — measures the average elongation of each 
one-inch section of the tested piece. Hence the product 



TESTING MATERIALS 105 

of Y m and AE\ — both expressed in inches — gives an 
area in square inches that is proportional to the ultimate 
resilience per cubic inch of the material. This value is 
obviously equal to the area of the diagram. Numeri- 
cally this value in inch pounds per cubic inch equals the 
average value of p in pounds per square inch, multiplied 
by the final value of e in inches per inch of test piece. 

A machine member may be subjected to shock in use 
and it is necessary to know the shock-resisting capacity 
of materials of machines. 

If a tensile shock were delivered to the piece whose 
test is recorded in Fig. 17, and if the energy of the shock 
were equal to the energy represented by the area ABiG, 
it follows that the shock would stress the piece to its 
elastic limit. Hence the area ABiG is proportional to 
the shock-resisting capacity of the material at the elastic 
limit. If s.-d. diagrams of different materials were plotted 
on the same scales, areas of triangles under the elastic 
limit could be compared to determine relative moduli 
of resilience or elastic shock-resisting capacity. 

Similarly, areas under the complete s.-d. diagrams meas- 
ure the capacity of the materials to resist rupture by shock, 
and comparison of these areas — in diagrams on the same 
scales — gives relative ultimate shock-resisting capacities. 

In Fig. 19 diagram I is a reproduction on an enlarged 
scale of the initial part of the s.-d. diagram of mild steel 
given in Fig. 17. Diagram II, Fig. 19, is the s.-d. diagram 
of stressed cast iron like that represented in Fig. 18. The 
elastic limit of the mild steel is at B, while the artificial 
elastic limit of the cast iron is at Bi. The modulus of 
resilience of the steel is proportional to the area ABC, 
while that of the cast iron is proportional to the area 
ABiCi. Obviously the steel has greater elastic resilience 
than the stressed cast iron. 



106 



MATERIALS OF MACHINES 



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Relative Elongation in inches per inch original length 

Fig. 19. 



All of the elastic properties of mild steel and stressed 
cast iron can now be compared by reference to Fig. 19. 



Strength at elastic limit of mild steel _ BC 

Strength at elastic limit of stressed cast iron BiCi 

for this case. 



= 1.86 



Since stiffness is proportional to - within the elastic 
limit, and since this ratio is constant within the elastic 



TESTING MATERIALS 107 

limit, it follows that any value of e less than AC may be 
chosen for the comparison; as for example, AK. 

KN 

Stiffness of mild steel = AK_ = KN = g5 
Stiffness of stressed cast iron KM KM 

AK 
for this case. 

The relative elongation at elastic limit is measured by 

+ u *• AC 
the ratio -t-tt ■ 
AC i 

Modulus of resilience of mild steel 

ron 

nran A TIP. 

= 1.8 



Modulus of resilience of stressed cast iron 

area ABC 
area AB1C1 

for this case. This measures the comparative shock- 
resisting capacity at the elastic limit of the two materials 
tested. 

The ultimate resilience of the stressed cast iron is pro- 
portional to the area AEG; while the ultimate resilience 
of the steel is proportional to the total area under the 
steel diagram which extends far beyond the limits of 
Fig. 19 (see diagram I, Fig. 17) . 

Compression. — In s.-d. diagrams of compressive tests 
unit stress is plotted downward and relative deformation 
is plotted toward the left. Fig. 20 shows diagrams of 
mild and high-carbon steel and of cast iron both in tension 
and in compression. Even if the strength of a given 
ductile material were the same in tension and compression, 
the compression diagram would have greater values of 
unit stress for the following reason: 

In a tension test, elongation is accompanied by reduc- 



108 



MATERIALS OF MACHINES 



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-d o 


\ 




























ffl 













































































<« p. 



(d 
• O 



o 4 ^ 



TESTING MATERIALS 109 

tion of section area, since the density remains unchanged. 
The true unit stress at any point of the test would be 
total tensile force divided by the corresponding section 
area; but it has been decided for engineering tests to use 
the original section area as a divisor throughout the test 
and hence all values of unit stress in tension are really 
too small. On the other hand, in a compression test the 
section area increases with reduction in length and 
hence all values of unit stress in compression figured on 
the original section area are really too large. In the 
comparison of tension tests with each other this would 
not lead to error, nor would it lead to error in case of 
comparison of compression tests with each other; com- 
parisons of tension with compression tests are probably 
unnecessary. This discrepancy in the s.-d. diagrams is of 
no importance within the elastic limit, since the change 
in section area up to that limit is negligible. 

A ductile material like mild steel fails under compressive 
force either by splitting parallel to its axis, or by flatten- 
ing out with a continually increasing section area, hence 
it is impossible to locate a definite breaking point. 

A brittle material fails in compression by shearing on 
planes at about 45 degrees to the axis of the test piece. 

S.-d. diagrams may also be plotted from the data of 
tests in which the external forces produce torsional or 
transverse stress as well as from the data of tensile and 
compressive tests. 



CHAPTER VII 

THE EQUILIBRIUM DIAGRAM OF IRON 
AND CARBON 

Certain pure chemical elements occur in two or more 
so-called allotropic forms whose physical properties are 
quite different. Thus, diamond, graphite and charcoal 
are allotropic forms of carbon; oxygen occurs in two 
allotropic forms, 2 and 3 (ozone). In general, change 
from one allotropic form to another is accompanied by 
absorption or release of energy. The change may be due 
to rearrangement of molecules or of atoms, or to some 
other unexplained cause. 

Iron has three allotropic forms called alpha (a) iron, 
beta (|8) iron and gamma (7) iron. Each form is stable — 
that is, it resists change into the other forms — within 
certain temperature limits. This is shown in Fig. 21. 
Temperature values in degrees Fahrenheit are laid off 
vertically upward. Horizontal spaces represent conven- 
tionally the time of heating or cooling. Between any 
attainable lower temperature and 1418° F. iron takes the 
form of solid a iron; at 1418° F. it changes to solid /? 
iron and holds this form until 1660° F. is reached, where 
it changes to solid 7 iron which melts at 2786° F. 

The diagram represents heating, but the changes begin 
at the same temperatures whether the temperature change 
is upward or downward; whether the iron is heated or 
cooled. Alpha iron is soft and ductile with tensile unit 
strength of about 40,000 pounds at air temperature. It 

110 



EQUILIBRIUM OF IRON AND CARBON 111 

is very magnetic, but loses this property entirely on chang- 
ing into |8 iron. The crystal structure of a iron is like that 
of j3 iron, but is quite different from that of y iron. A 



2800 
2600 
2400 
2200 
2000 
.1800 
















Liqui 


1 Iron 




/ 


/ 
















11 


278 


5° / 


















Solic 


1 ' 


' It 


on / 
























/ 


/ 
























/ 






















/ 


/ 
























/ 


\ 


t 










£1600 

|1400 
H 

1200 
1000 
800 
600 
400 
200 
n 






t 
















1 


418° r _ 




S 


1 
alidQ 


[ron 
















































Solid t 


: Iron 






































































































60°/ 

























HEATING CURVE OF PURE IRON 
Fig. 21. 



certain small amount of heat becomes latent during the 
change of a. to /3 iron, and a larger amount becomes latent 
during the /3 to y change. These and other facts show 



112 MATERIALS OF MACHINES 

that a, jS and y iron are allotropic forms with distinct 
physical differences. 

Liquid iron will take freely into liquid solution many 
metallic elements, such as manganese, nickel, cobalt, 
chromium, tungsten, vanadium and copper, as well as 
non-metallic elements, such as silicon, carbon, phosphorus 
and nitrogen; certain quantities of these remain in solid 
solution when the iron solidifies in the 7 form. When 
the cooling progresses into the /3-iron field, though the 
metallic elements are probably held in solid solution, 
the non-metallic elements seem not to be retained. With 
further cooling into the a-iron field, moderate amounts of 
the metallic elements are held in solid solution, while the 
non-metallic elements are held only slightly or not at all. 

Carbon is the most important element whose presence 
exerts a modifying influence upon iron. It may be asso- 
ciated with solid iron in solution, in chemical combination 
or in mechanical mixture. The phenomena accompanying 
changes of temperature of associated iron and carbon 
can be best explained by use of the so-called equilibrium 
diagram. Such a diagram is given in Fig. 22; it is drawn 
only with approximate accuracy. Temperatures are 
laid off vertically upward on the axis of F, and percentages 
of carbon present with the iron are laid off horizontally 
toward the right on the axis of X. At A is the solidifica- 
tion temperature of pure iron and solidification becomes 
complete at this temperature; but as carbon is added to 
the iron the solidification begins at lower temperatures, as 
shown by the line AD. Moreover, while pure iron be- 
comes completely solid at the temperature A, associated 
iron and carbon begin to solidify at temperatures defined 
by AD, but solidification does not become complete 
until some lower temperature, defined by AB, is reached. 
For illustration, the point h represents liquid iron contain- 



EQUILIBRIUM OF IRON AND CARBON 113 



3000 


e 


/ g 


h 




N 




i 




n 






A | 


1 1 

1 i 


1 


















E 


2800 
2600 
2400 






1 
















711 / 


/ 




H 

i r 


\ L 


^2 






LIQUID SOLUTI 


)N 


A 


quid 




i i 

1 1 

i i 


?3\ 


7 (-G-) 




p^tta 


A 






h 


'+G (i 

Fe 3 C 


table) 
able)— 

F 


2200 

2050 
2000 

1800 

^'1660 
£1600 

Pi 


~+~u 


uid 






i i 

'(<?) 1 

1 


1 

1 

I 




B 




h 




*^D 


/ 


h 






















1 


/ 


















\K 


J 

-J(C| 


!/ 


/ 






7(C) 
+ F« 


l-G (st 

> 3 C (B 


able) 
Qstable 








5 


1 


p/ii 


















R 




/ 




7 


C)+Fe 


, C (stable) 










©1400 


L 


*ty 








+ G ( unstable} 








S 


1200 
1000 
800 
600 
400 
200 



M \ 


°l 






















a 


+ 7(CJ) 
| 














































*e— 


— I 




x + Fe 


C (St£ 


ble) + 


G (un 


3table) 








-— 




1 
1 
























1 

1 
1 
























1 

0.9 J 






Ni 
















0. 


5 ] 


l~J 


5 % 


2 
Perc 


5 J 

ento 


I 3 
ECart 


5 4 
>QH 


- i 


5 I 


> 5 


5 6 



Fig. 22. 



ing about 1| per cent of carbon at a temperature of 
3000° F. During cooling the point moves from h vertically 
downward, and when it reaches d on the line AD solidifica- 
tion begins, but is not completed until the point moving 
vertically — with falling temperature — reaches b on the 
line AB. 



114 MATERIALS OF MACHINES 

The first crystals that form do not contain the propor- 
tions corresponding to d, but a smaller amount of carbon; 
in fact the composition is indicated by the point b 2 , where 
a horizontal through d cuts A B. As d descends, the com- 
position of the forming crystals is continuously indicated 
by the intersection with A B of the horizontal through 
the moving point; thus with the point at d\ the composi- 
tion of the forming crystals is indicated by b\, at the 
same time the composition of the residual liquid, which 
grows constantly richer in carbon, is indicated by d 2 where 
the horizontal cuts AD. Thus crystals form continuously 
with increasing carbon content and this results in formation 
of crystal groups with cores low in carbon surrounded by 
layers of crystals having increasing carbon content. 

Now, since the first-formed crystals have low carbon 
and since the average carbon content cannot change, it 
follows that the later-formed crystals must be high in 
carbon, and hence the outermost layers of the crystal 
groups must have carbon content much above the average 
value, and hence the point 6 2 on A B, instead of stopping 
at b, continues toward B to some point that holds the 
average at b. 

Obviously the solid cannot be homogeneous unless it 
is made so during or after solidification by redistribution 
of carbon by diffusion. Diffusion through the solid is a 
slow process and hence cooling must be very slow through- 
out the solidification range and afterwards if a homo- 
geneous solid is to result. 

In case of steel ingots, which are "stripped" and 
transferred to " sokking-pits " for slow cooling and heat 
equalization while the ingot interior is still liquid, un- 
doubtedly there is time for diffusion of carbon to pro- 
duce uniform distribution. But in steel castings the 
cooling is too quick to permit this result and there is, 



EQUILIBRIUM OF IRON AND CARBON 115 

as micro-photographs show, a lack of uniformity of carbon 
content in the crystals. 

While the point is above AD the liquid iron holds the 
If per cent "carbon in liquid solution; between AD and AB 
the carbon is partly in liquid and partly in solid solution, 
and below AB the mass is solid, the carbon being in solid 
solution in the iron. The results of cooling will be similar 
for any amount of carbon up to about 2.1 per cent, or 
with the cooling point located anywhere above AD and at 
the left of NNi. Iron will hold carbon in liquid solution 
in amount according to temperature, as shown by the 
limiting line DE. Thus at 2600° F. iron will hold 5.4 per 
cent carbon corresponding to the point M; at 2800° F. 
the liquid solution is saturated with 5.75 per cent carbon; 
see point E. But 2.1 per cent seems to be about the limit 
of carbon in solid solution in iron. Therefore, if the 
molten mass contains more than 2.1 per cent carbon the 
excess must separate on solidification. This may be 
illustrated on the diagram. The point j represents 
3 per cent carbon in liquid solution in 97 per cent iron at 
3000° F. As this liquid cools the point moves vertically 
downward and when j h on AD, is reached, solidification 
begins; but the solid crystals first formed can only contain 
the amount indicated by 6 3 , about 1.3 per cent carbon, 
and hence it must have given up the residue of its original 
3 per cent which is added to the residual liquid whereby 
that liquid is enriched in carbon. Thus the cooling point 
representing the composition of the residual liquid 
moves toward the right; in fact it moves along the line 
AD. Meanwhile the point representing the composition 
of the crystals moves along the line AB from 6 3 toward B 
and the residual liquid grows richer in carbon until the 
cooling point that started from j\ reaches j 2 ; the residual 
liquid composition corresponds to D and the mass solidi- 



116 MATERIALS OF MACHINES 

fies without further drop in temperature. The product 
of this final solution is 7 (C), associated with whatever 
is crowded out during the entire change from liquid to 
solid. Since, by assumption, the other substance present 
is carbon, it follows that only carbon or compounds of car- 
bon and iron could be associated with the 7 (C) . The 
solidified mass really does consist of intimately associated 
crystals of 7 (C), carbon as graphite, G, and iron carbide, 
Fe 3 C. 

But the starting point of cooling might have been at 
some point at the right of nD, for example, I, representing 
a liquid solution of 5 per cent carbon in iron. The point 
I, moving vertically downward during cooling, reaches the 
line DE where the liquid iron is saturated with carbon 
and where further fall in temperature results in separa- 
tion of solid crystals, probably Fe 3 C *, which may change 
in part to graphite in the effort to establish equilibrium. 
The remaining liquid is thus impoverished in carbon and 
hence the point representing composition of residual 
liquid moves toward the left; in fact it follows the line 
ED to D, and during this progress Fe 3 C separates continu- 
ously. At D the residual liquid contains 4.5 per cent 
carbon in liquid solution, and during final solidification 
at constant temperature the excess of carbon over 2.1 
per cent is crowded out as Fe 3 C and graphite. The 
product of solidification at D from initial condition j and 
at I are similar, but of course with different proportions 
of 7 (C) and G .+ Fe 3 C. 

With n as the starting point of cooling the liquid con- 
sists of a 4.5 pier cent solution of carbon in iron and the 
cooling line cuts AD and ED at their intersection D. 

* It seems to be accepted as a fact that when two forms may 
separate from a solution the form that is unstable under the con- 
ditions separates first. 



EQUILIBRIUM OF IRON AND CARBON 117 

Hence no preliminary separation of 7 (C) occurs as with 
the line of cooling at the left of nD, nor preliminary sepa- 
ration of Fe 3 C and graphite as with the line of cooling at 
the right of nD, but the solution remains constant until 
D is reached, and solidification begins and ends at that 
temperature. Before solidification the mass is all liquid 
solution with 4.5 per cent carbon; after solidification 
the iron holds 2.1 per cent carbon in solid solution, but 
the excess of carbon above 2.1 per cent separates as ce- 
mentite (unstable) or graphite (stable). Hence the solid 
product of cooling from n consists of intimately mixed 
crystals of 7 '(C), Fe 3 C and G with the total carbon present 
equal to 4.5 per cent. This solid is called "eutectic," 
and D is the " eutectic point." 

With cooling fromj the solid product consists of eutectic 
formed at D mixed with excess of 7 (C) with varying 
carbon content formed during temperature change from 
ji to ji. With cooling from I the solid product consists 
of eutectic formed at D mixed with excess of G and Fe 3 C 
formed during temperature change from h to l 2 . 

Certain changes in composition of associated iron and 
carbon also occur as temperature falls after complete 
solidification. 

The lines KNO, BO, LN and MO (Fig. 22) have been 
located on the diagram by careful experiments. At K 
is the temperature of interchange between the (3 and 7 
forms of pure iron. The sloping line KN indicates the 
drop in temperature of (3, 7 interchange due to increasing 
proportion of carbon. At L is 1420° F., the temperature 
of interchange between a and /? forms of pure iron, and 
between L and N any free iron in the mass would inter- 
change between (3 and a at the constant temperature LN. 
The sloping line NO indicates the drop in temperature of 
a, 7 interchange due to increasing proportion of carbon. 



118 MATERIALS OF MACHINES 

The line BO defines the lowest temperatures at which 
7 (C) solid solutions can exist without separation of 
cementite or graphite, or it is the line of solid saturation. 

First illustration. — The point e represents a solution 
of 0.2 per cent carbon in liquid iron at 3000° F. As this 
point moves vertically downward on the line eei during 
slow cooling, it passes the line AD where solidification 
begins, and the line AB where solidification ends. The 
point continues to move vertically downward, the cooling 
material remaining 7 (C) and with slow cooling probably 
becoming homogeneous by diffusion, until the line KN 
is reached at Cij at this point some of the iron changes 
from the 7 (C) to the /3 form, and since /3 iron cannot 
hold carbon in solution, the 7 (C) that is thus left is 
impoverished in iron; hence the cooling point moves in 
the direction of increased carbon toward the right; in fact 
it moves along the line KN until the point N is reached; 
at this temperature, 1420° F., the /3 iron that has sepa- 
rated out as pure iron changes — as any pure iron would 
at that temperature — into the a form, while more iron 
separates from the 7 (C) as a iron, causing further iron 
impoverishment of the remaining 7 (C) ; then the cooling 
point moves along the new slope, NO, until the point O is 
reached at the lowest temperature at which iron can exist 
in the 7 form, and the residue of 7 (C) changes into a 
eutectic mixture called pearlite, consisting of a iron and 
Fe 3 C. 

Second illustration. — Let / be the starting point of 
cooling with 0.7 per cent carbon. As before, the cooling 
point passes the lines AD and AB, where solidification of 
7 (C) is completed, and arrives at /1 on the line NO, where 
7 iron changes into a iron (possibly passing through the 
intermediate /3 form) impoverishing the remaining 7 (C) 
in iron and causing the point to move along the line NO 



EQUILIBRIUM OF IRON AND CARBON 119 

to the point where the eutectic pearlite is formed as in 
the first illustration. 

Third illustration. — With g as the starting point of 
cooling the carbon equals 0.9 per cent, which is just the 
proportion corresponding to the eutectic; the point goes 
vertically downward passing AD and AB, the range of 
solidification, and continuing to where, since this is 
the eutectic point, the y (C) changes directly into the 
eutectic pearlite. 

Fourth illustration. — With h as the starting point of 
cooling, the carbon equals 1.25 per cent and the point 
moves through the solidification range and finally reaches 
hi on the line BO. Here the condition is that of a solid 
saturated solution of carbon in iron and hence further 
reduction in temperature results in separation of Fe 3 C. 
The result is that the remaining 7 (C) is impoverished 
in carbon and hence the point must move to the left; 
it really does move along the line BO and finally reaches 0, 
where the 7 (C) changes into the eutectic pearlite as before. 

If the cooling point starts at the left of g, iron separates 
from the 7 (C) along the line KNO to leave the eutectic 
proportion at 0, and the resulting mass consists of crys- 
tals of a iron intermixed with eutectic pearlite. 

With g as the starting point of cooling all 7 (C) changes 
directly into eutectic and hence the resulting mass will 
consist of the eutectic alone. 

With the starting point of cooling at the right of g, 
Fe 3 C separates from the 7 (C) along the line BO to leave 
the eutectic proportion at 0, and the resulting mass con- 
sists of eutectic pearlite with crystals of Fe 3 C. 

Upon the fields of the diagram, Fig. 22, are given the 
forms there taken by associated iron and carbon. Thus 
above the lines AD and DE there can be only liquid solu- 
tion. To the right of DE (since the line DE gives the 



120 MATERIALS OF MACHINES 

saturation limit) carbon in excess of saturation must exist 
separately — usually as graphite — in fact in this field 
graphite separates by gravity and floats on the surface, 
thus carrying the residual mass back to the saturation 
line. Fe 3 C is also present in this field since it separates 
first and since change into the more stable graphite occurs 
very slowly. 

Cooling lines from the field ADE enter the field FBPR 
through the eutectic point D, carrying eutectic with 
amounts of graphite and cementite that vary according 
to the location of the starting point of the cooling curve. 

The line PR has been carefully located where graphite, 
G, and cementite, Fe 3 C, interchange stability. Above this 
line cementite tends to rapid change into graphite (giving 
up its combined iron), while below this line graphite tends 
to become cementite slowly, taking up the required iron. 
Below the line MS iron cannot exist in the 7 form, but 
changes into the a form, carrying graphite (unstable) and 
cementite (stable). 



CHAPTER VIII 
CAST IRON 

The blast-furnace produces different grades of pig 
iron that are usually numbered from 1 to 6, although 
the numbering scheme varies in different countries and 
in different localities in the same country. When the 
product of the blast-furnace has been melted in a foundry 
cupola, or a reverberatory furnace, cast into sand molds 
and cooled, it is called cast iron. 

Cast iron usually contains carbon, silicon, manganese, 
sulphur and phosphorus; its physical properties differ 
from those of pure iron because of the presence and inter- 
action of these substances. 

Carbon is usually present in cast iron in two forms: 
(a) as pure carbon in the form of distributed crystals of 
graphite, and (6) as combined carbon in the form of dis- 
tributed crystals of cementite, Fe 3 C. These forms are 
usually called graphite and combined carbon. 

Cementite is an intensely hard and very brittle sub- 
stance, and the influence upon iron of a very small propor- 
tion of carbon in this form is very great. This is partly 
due to the fact that a small weight of carbon makes a 
large weight of Fe 3 C. Taking the atomic weight of carbon 
as 12 and of iron as 56 the ratio of weight of cementite 

to the contained carbon equals — ^ = 15; hence 

iron that has one per cent by weight of combined carbon 
may really have 15 per cent by weight of Fe 3 C. Since 

121 



122 



MATERIALS OF MACHINES 



the specific gravity of iron is 7.8 and of Fe 3 C is 7.07, it 
follows that the volume of Fe 3 C in the case just cited is 
16.3 per cent. 

Percentages by weight and by volume of Fe 3 C in iron 
with varying carbon may be tabulated as follows: 



Per cent weight 


Per cent weight 


Per cent volume 


carbon 


Fe 3 C 


Fe 3 C 


1 


15 


16.3 


2 


30 


32.1 


3 


45 


47.4 


4 


60 


62.3 


4.5 


67.5 


69.6 



These values result from the assumption that only iron 
and carbon are present and they would of course be modi- 
fied in the case of actual cast iron; but they show how 
great an influence a small amount of combined carbon 
may have on cast iron. 

Since cementite is hard and brittle, its presence in large 
proportion gives similar qualities to cast iron. In fact, 
white cast iron, in which carbon is chiefly in the combined 
state, is weak in tension, though strong in compression; 
it is so hard that it can only be machined by grinding; 
it is also brittle, that is, it has very low ductility and 
resilience; but it has great resistance to wear and hence 
it is produced by "chilling " in the surfaces of rolls for 
rolling steel and in the treads of cast-iron car wheels. 

Graphite is an allotropic form of carbon and when it 
is present in cast iron it produces a gray fracture and inter- 
rupts the continuity of the iron structure and thus re- 
duces the strength of the iron with which it is associated. 

Conversion of the carbon of the Fe 3 C of white cast iron 
into the graphite of gray cast iron leaves the iron of the 
Fe 3 C (which may be a large, proportion of the mass) in 



CAST IRON 123 

the a form and thus substitutes a ductile material strong 
in tension for a brittle one; moreover, both the iron and 
graphite are soft and hence the change from white iron to 
gray iron, except in extreme cases, is accompanied by in- 
creased tensile strength and ductility and softness; the 
compressive strength is decreased. Hence means for 
control of carbon between the states of cementite and 
graphite are of great importance. 

The total carbon in cast iron is almost always within 
the limits 2.6 per cent and 4.6 per cent; therefore, in 
Fig. 23, the lines QQi and HHi bound the cast-iron field 
on the equilibrium diagram. 

Molten iron associated only with carbon, ready for 
casting, would be represented by some point in the field 
above qDr at (say) 2400° F. If, during melting in contact 
with carbon fuel, the iron had become saturated with car- 
bon — as often occurs — the cooling point would be at 
w and during cooling either in the ladle or mold, Fe 3 C 
would separate and with sufficient time the carbon would 
change to graphite which would float as long as the mass 
remained fluid enough for gravity to cause the separation, 
and afterward would be irregularly distributed throughout 
the mass. Thus the cooling point would follow wD to D 
where the eutectic* would be formed, and this eutectic 
would be more or less uniformly mixed with the graphite 
that separated along wD through the Fe 3 C form. Doubt- 
less also some Fe 3 C would remain unconverted. 

As very slow cooling goes on the line PR is reached 
where the stability of G and Fe 3 C interchange, and there 
is a tendency for the graphite formed to take up iron and 

* X (C) with 4.5 carbon in liquid solution changing into crystals 
of X (C) with 2.1 per cent carbon in solid solution, intimately as- 
sociated with crystals of graphite corresponding to the residue of 
the 4.5 per cent carbon. 



124 

3000 
2800 
2600 
2400 

2200 

2050 
2000 

1800 

^1660 
£1600 

P. 



MATERIALS OF MACHINES 



a 



1400 



H133Q 
1200 

1000 

800 



400 



200 













Q 










H 






A 






















E 


^ 


\ 


«^. 






















\ 


y 








LIQUID SO 
v t 


.UTior 

s 


si 

1 




iiq.Sol. 
stable) 








s y 


Q 
C)+ L 


i j.Sol. 


"l 






/ 


4-Fe 3 C 

(UI 


stable) 










B 








"^D, 


/'■ 




F 








* 






















7(C 


)+ 


3 (stable) 








K 












+ 


Fe ; 


C( unstable 








\ 


i 












\ 










R 










7(C)+ 


*Fe : 


C (stable 








L 


^, 












\ 


r + C 


3 (unstah 


lb) 




S 


M 

















k 
















































a- 


hFe 


•:C 


(stable) 




















' 


hG 


(ur 


stable) 






































































Qi 




\ 


/ 




Hi 









5 ] 


L 1 


5 2 


2.1 

Per< 


sent c 


1 3. 
f Carl 


5 

JOI 


i 
1 


L 4.5 


£ 


5. 


5 6 



46 



Fig. 23. 



to become Fe 3 C. This tendency, however, is less if a large 
amount of graphite is present and especially if it is present 
in large flakes. At the line MS, since this represents the 
lowest temperature at which y iron is stable, y (C), with 
slow cooling, tends to change into a iron and Fe 3 C; but 
here also the presence of graphite, especially in large 



CAST IRON 125 

flakes, tends to oppose the change and may compel the 
decomposition of y (C) into a iron and temper graphite.* 
In this case the resulting mass will have with the a iron a 
large amount of graphite as flakes and temper graphite 
and a relatively small amount of cementite. It would 
therefore be a gray iron. 

But suppose that the cooling point instead of starting 
from w should start from some point v corresponding to 
3 per cent of carbon and 2400° F. The point would de- 
scend vertically to Vi and follow vj) with separation of 
solid y (C), which would be more or less uniformly dis- 
tributed throughout the mass; on reaching D the residual 
liquid would be decomposed into the eutectic (as before 
intimately associated y (C) and G) . The resulting mass, 
therefore, would consist of eutectic with excess of y (C), 
and with very slow cooling, in the absence of excess of 
large flake graphite, on reaching PR the graphite wholly 
or in part would take up iron and become Fe 3 C. Hence 
between PR and MS the mass consists of y (C) + Fe 3 C + 
some graphite. At MS the y (C) is decomposed (in the 
absence of large flake graphite) into a iron and Fe 3 C. 
This mass would have large excess of Fe 3 C and little 
graphite and hence would be a white or light gray iron. 

Chilling. — The changes just considered are accom- 
plished slowly and hence rapid cooling might render them 
incomplete, while sudden cooling might suppress them 
entirely. If molten cast iron were poured into an ample 
bath of cold water, cooling would be sudden and the cooled 
iron would have a large proportion of its carbon in solu- 
tion, y (C), because of lack of time for the change, while 
a small proportion would be combined in Fe 3 C. Between 
this extreme of sudden cooling and the other extreme of 

* Temper graphite is graphite very finely divided; in fact, it is a 
microscopic dust, uniformly distributed as in malleableized castings. 



126 MATERIALS OF MACHINES 

very slow cooling, the cooling rate can be varied with the 
production of intermediate results. 

In casting chilled car wheels, the portion of the mold 
that forms the tread is of iron, which conducts heat away 
rapidly, while the portion that forms the rest of the wheel 
is of sand, which conducts heat away slowly. Thus the 
tread is " chilled" and, with proper grade of iron, cools 
white, while the rest of the wheel is gray, with an inter- 
mediate territory of mottled iron. This gives a wheel 
with a very hard tread to resist wear, a strong web and a 
hub that is not only strong but soft for machining. See 
Fig. 24. 



Jr'l' -^^H 










Pl^^^SiS 


-A 


j 


Iron. %. 










^-(lr4S Irfftv. 




X) 



Fig. 24. 

The rate of cooling of castings made in sand molds 
varies. The rate of outflow of heat is proportional to 
the external surface of the casting, while the heat to be 
removed to effect cooling is proportional to the volume of 
the casting; hence the rate of cooling depends upon the 
ratio of surface to volume of the casting, and it follows 
that thick castings cool more slowly than thin castings. 



CAST IRON 127 

Hence the same iron cast into thick and thin forms would 
give castings of different composition, structure and 
properties. 

If it is desired to accelerate cooling, the sand of the mold 
which acts as heat insulation may be removed as soon as 
the iron is cool enough to hold its form. 

The form of carbon in cast iron may be controlled by 
regulation of the quantities of other substances present. 

Manganese is held in solid solution by iron in a wide 
range of proportions; it also may combine chemically 
with carbon to form a carbide, Mn 3 C, which goes into 
solid solution with Fe 3 C to form what may be called 
iron-manganese carbide. The carbide Mn 3 C is hard and 
brittle and its association by solution with Fe 3 C, which 
is only a little less hard and brittle, would presumably 
produce a compound with similar qualities, which it 
would give in less degree to the mass with which it might 
be associated. Moreover, since the manganese must take 
up carbon to produce the carbide it would reduce the 
carbon which, in its absence, would appear as graphite; 
thus gray iron would become whiter. 

Authoritative tests seem to show that addition of man- 
ganese to mild steel increases tensile strength, and that 
ductility, though little changed up to 0.5 per cent, is 
reduced with further additions of manganese until at 
2 per cent the steel is quite brittle. Since there is a very 
small amount of carbon in this mild steel, it is probable 
that these effects are due to manganese in solution with 
the iron itself. 

Whatever the theory may be it is certain that manga- 
nese up to 1 per cent in cast iron increases strength and 
probably reduces ductility, and that in larger amounts it 
tends to change graphite into combined carbon. 

If sulphur is present in cast iron its form is FeS, and 



128 MATERIALS OF MACHINES 

it has a very decided tendency to cause graphite to become 
combined carbon. This tendency of sulphur is much 
greater than that of an equal amount of manganese to 
produce the same result. When manganese is added to a 
cast iron containing sulphur it takes sulphur from the FeS 
to form MnS. Consider now three substances: (a) iron 
sulphide, (6) manganese sulphide, (c) iron manganese car- 
bide, (a) has greatest power to convert graphite into 
combined carbon; (c) is next in order and (6) has least 
power to produce this result. With a given amount of 
FeS present the addition of manganese in amount that 
would just take all sulphur to form MnS, would convert 
a substance, FeS, with great power to make gray iron 
white into a substance, MnS, with small power for this 
result; and thus the graphite would increase at the ex- 
pense of combined carbon and the iron would become 
grayer. Further increase in manganese would now pro- 
duce an increasing amount of iron manganese carbide 
with increasing tendency to make the gray iron white, 
until finally this tendency would just equal that of the 
sulphur before any manganese was added and further 
increase would make the iron whiter still. Obviously, 
the limits of this reversed action depend on the amount 
of sulphur present. With no sulphur, the effect of increas- 
ing amount of manganese would be increasing tendency 
to whiten the iron; whereas when sulphur is present, the 
effect of increasing amount of manganese would be first 
to neutralize the sulphur with increasing grayness of the 
iron, and then to turn the gray iron whiter. This ex- 
plains why a small addition of ferromanganese to molten 
sulphur-iron may act as a softener. Sulphur is undesirable 
in cast iron, causing hardness, weakness and brittleness; 
it should not exceed 0.15 per cent and thus it should not 
be used as a means for regulation of the state of carbon. 



CAST IRON 129 

Manganese, on the other hand, within limits has a desir- 
able effect upon strength and hence may be used to reg- 
ulate carbon. 

Silicon is held in solid solution by a iron up to about 
2 per cent and up to this limit it seems to increase the 
strength and to reduce ductility of iron; beyond this limit 
Fe 2 Si forms in increasing amount, and the presence of 
this silicide reduces strength, ductility and shock resist- 
ance of cast iron. Silicon also tends to reduce iron oxide, 
to remove gas that causes porosity and to increase the 
fluidity of the molten iron, so that stronger, denser and 
sharper castings are produced. Silicon also tends to 
force carbon from the combined to the graphitic form, 
or to make iron grayer. Possibly silicon that forms Fe 2 Si 
takes the iron for this purpose from Fe 3 C, thus leaving 
the carbon to take graphitic form. Obviously, silicon 
would tend to neutralize the effect of manganese and 
sulphur to make iron whiter. 

Ferrosilicon, as high silicon pig iron is called, is now on 
the market, and its use in the foundry cupola charge gives 
control of the silicon in castings and thus within limits 
gives control of the state of the carbon in the cast iron. 

To determine the effect of a varying proportion of silicon 
upon cast iron, Professor Thomas Turner made a series 
of experiments which were reported under the title " In- 
fluence of Silicon on the Properties of Cast Iron " in the 
Journal of the Chemical Society in 1885. He used iron 
with as nearly as possible 2 per cent carbon and with 
sulphur, phosphorus and manganese quite low, and by 
introducing ferrosilicon he was able to produce test 
pieces with desired proportions of silicon. These were 
tested and results in tensile and compressive strength 
and hardness are plotted in Fig. 25. 

Progress from toward the right, with increasing 



130 



MATERIALS, OF MACHINES 



tensile strength and decreasing hardness, probably cor- 
responds to the combined effect of solution of silica in 
iron, and the influence of formation of Fe 2 Si to increase 
graphite at the expense of combined carbon. 

























\ 




















X 

V 






































V 




















\o 






^ 


t& 












V 


i 












y 


















































































0£j^r 


1SILE 


3TRENC 


5TH 











200 



180 



160 



140 < 

a 

CO 



Q. 
CO 
Q 

100 i 

2 

80 g 
< 

CO 



60 



40 



5 6 7 

Fig. 25. 



9 10— # SILICON 



As has already been stated, white iron is stronger in 
compression than gray iron, and hence the compression 
curve should fall from the start whereas it rises up to 
about 0.8 per cent. This may be due to the direct ac- 
tion of the silicon upon the iron or to the fact that small 
amounts of silicon tend to reduce the porosity that is 
common in white cast iron and to give sounder and 
hence stronger castings. 



CAST IRON 131 

It is clear from these tests that by varying the silicon 
content it is possible to control very materially the physi- 
cal properties of cast iron. It would be very helpful to 
know what the effect of increasing silicon would be with 
the total carbon higher than 2 per cent to correspond 
more nearly to foundry practice. 

Phosphorus probably goes into solid solution in iron 
in amounts such as are usually present in steel; but as it 
increases to the values common in cast iron a large part 
of it becomes FesP. Phosphorus may affect cast iron in 
three ways: (a) directly, by solution in the iron; (6) indi- 
rectly, by its power to cause graphite to become combined 
carbon; (c) indirectly, because its presence causes cooling 
iron to pass through a pasty state thus delaying solidifi- 
cation. This delay makes it possible for tendencies that 
are active in the cooling mass to produce more complete 
results. Thus with high silicon the presence of phos- 
phorus — by delaying solidification — might enable the 
silicon to produce a larger proportion of graphite, thus 
giving a softer iron stronger in tension in spite of the op- 
posite effect of the phosphorus itself. Obviously the re- 
sultant effect would depend on the relative amounts of 
the silicon and phosphorus present and with low silicon 
and high phosphorus the effect would undoubtedly be to 
whiten the iron, (a) and (b) produce harmful increase in 
hardness and brittleness; (c) in the presence of silicon 
may produce a desirable increase in strength and soft- 
ness. Again, whatever theory may be right, it is a fact 
that although phosphorus is a very detrimental constitu- 
ent in steel, it is not harmful in cast iron, often being 
present up to 1.3 per cent. Phosphorus is useful in cast 
iron because it increases the fluidity of the molten mass 
so that sharp castings can be made with lower casting 
temperature. 



132 MATERIALS OF MACHINES 

Semi-steel. — There is another method of carbon 
control that gives excellent castings. In case of cast iron 
with low total carbon, if all or nearly all of the carbon 
could be caused to appear as very finely divided graphite 
— like temper graphite in malleableized castings — there 
would be very low cementite and relatively small inter- 
ference with the continuity of the iron by the graphite, 
and hence the iron would be strong and soft. 

In many foundries it is now customary to introduce 
with the regular cupola charge about 25 per cent of steel 
scrap, which mixes with the cast iron and melts. Ne- 
glecting the carbon of the steel, and assuming the total 
carbon of the rest of the charge to be 4 per cent it follows 
that the total carbon would be reduced by dilution to 
3 per cent. Or in Fig. 23 the starting point of cooling 
would be moved from s to v. Cooling slowly from v with 
only carbon and iron present would give a white iron. 
Addition of sulphur, phosphorus and manganese in cus- 
tomary amounts would leave the iron still white; but 
addition of silicon in sufficient quantity could cause most 
of the carbon to appear as finely divided graphite (since 
the total carbonjs low), giving a soft strong iron. It is 
interesting to note that there is a possible ideal relation 
of quantity of these substances; viz., with a certain neces- 
sary amount of sulphur there should be enough manga- 
nese to convert all of the FeS to MnS. The total carbon 
should be kept low enough by dilution so that when the 
greatest possible proportion is converted into graphite, 
it shall appear as very fine grains instead of as flakes. 
The amount of silicon should be sufficient to provide for 
the fluxing that removes iron oxide completely and pre- 
vents porosity, and also by solution or chemical combi- 
nation to cause the maximum change of combined carbon 
into finely divided graphite. There should be just enough 



CAST IRON 133 

phosphorus to delay solidification for the silicon to produce 
its best result without too great phosphorus effect on 
carbon and iron. It would probably be impossible to 
produce this ideal result, but it is true that the so-called 
semi-steel often has tensile strength above 30,000 pounds 
per square inch, together with close grain and good wear- 
ing resistance and softness for easy machining. 

Aluminum introduced into molten cast iron produces 
two results. A part combines with oxygen to form alu- 
mina, thus reducing undesirable oxide of iron and absorb- 
ing gas that would cause porosity. The alumina thus 
formed combines with other waste and forms either a 
fusible slag or an infusible crust which is removed. An- 
other part of the aluminum — if there is an excess — may 
combine with the iron, and when it does, as shown by 
Mr. J. W. Keep,* its influence upon the distribution of 
carbon is similar to that of silicon. But although ferro- 
aluminum is on the market, it is not used as a softener of 
cast iron because it is more expensive than ferrosilicon, 
which produces the same result; moreover, when alu- 
minum is used, a skin that forms on the surface of the 
molten iron tends to cause "cold shuts " and defective 
casting surfaces. 

Malleable cast iron. — Theory of mallifying process 
for production of malleable cast iron. 

An average composition of castings for mallifying is as 

follows: 

Per cent 

Total carbon 2 . 75 

Silicon 0.8 

Manganese 0.4 

Phosphorus 0.17 

Sulphur, under . 05 

* See Transactions Am. Inst. Mining Engineers, Vol. XVIII, 
p. 102. 



134 MATERIALS OF MACHINES 

These castings with low carbon, low silicon and relatively 
high manganese and phosphorus, and with the relatively 
quick cooling which corresponds to malleable iron foundry 
practice, hold all carbon in the combined state; that is, 
the casting fractures white. In the mallifying process the 
castings, packed in iron oxide or other material, are raised 
to a temperature of from 1500° to 1600° F. This brings 
them into the lower portion of the field (Fig. 23) bounded 
by the lines QQ h HHi and BF, PR, where graphite is 
stable and where Fe 3 C is unstable. In passing the line 
OS upward the a iron changes into y iron and takes carbon 
into solution to form 7(C); but the amount of carbon 
available is small, since Fe 3 C is stable in this field, and 
hence probably 7 (C) holds less than 1 per cent of carbon 
in solution. The castings are held in the field above PR 
for about 60 hours, and this time is sufficient for the 
stability tendencies to reach equilibrium, and most of the 
Fe 3 C gives up its carbon to form temper graphite which 
appears as uniformly distributed microscopic dust.* The 
iron thus isolated in this field by decomposition of Fe 3 C 
is in the 7 form, and it takes silicon and carbon into solu- 
tion, forming 7 (Si, C). If no silicon were present 7(C) 
would be formed, but when silicon is present it crowds out 
and replaces a part of the carbon of 7 (C) giving 7 (Si, C). 
Hence the greater the amount of silicon within limits the 
greater the amount of carbon that appears as temper 
graphite. The result, therefore, of holding the castings 
in this field is production of 7 (Si, C) with low carbon, 
associated with temper graphite and, undoubtedly, with 
a small amount of unchanged Fe 3 C. 

* Probably the carbon takes this form here because the mass is 
resistant to the migration of the carbon. When the eutectic is formed 
at D, the solidifying mass, being less resistant, permits the separating 
graphite to migrate through the mass and to unite into graphite 
flakes. 



CAST IRON 135 

When the slow cooling takes place, on passing OS the 
7 iron of the 7 (Si, C) changes back to a iron, releasing 
the carbon but holding the silicon in solid solution, a (Si).* 
The released carbon in the presence of the a (Si) and of 
the temper graphite already formed is forced, in opposition 
to the equilibrium tendency of this field, to become temper 
graphite, and thus the mallifying process is complete and 
the cooled castings are strong and ductile with a fracture 
that is black with temper graphite with a thin white skin. 
The reasoning given applies only to the interior portion 
of the castings, since the white skin that is formed on the 
castings during mallifying has its total carbon notably 
reduced by oxidation either by the oxygen of the air 
trapped in the packing, or by the oxygen of the oxide 
packing itself, thus becoming a skin of something like 
mild steel. The ductility of this skin is of great impor- 
tance in the malleable castings, and its removal sensibly 
diminishes their value as stress members of machines; 
hence the stress-parts of malleable castings are seldom 
machined. 

The " white heart " malleable castings of England and 
the continent of Europe consist of light or very thin cast- 
ings that are almost completely decarbonized, like the skin 
of the American "black heart " castings, by higher malli- 
fying temperature and extension of the time of mallify- 
ing. 

Shrinkage. — When molten cast iron is poured into 
a mold it takes the form of the mold and cools gradually 
to the temperature of the surrounding air. Shrinkage 
which accompanies cooling may be divided into fluid 
shrinkage and solid shrinkage. 

As the molten iron in the mold after casting begins to 
Cool, it shrinks in volume. This shrinkage may be "fed 

* Some silicon is also in solid solution with the remaining Fe 3 C. 



136 MATERIALS OF MACHINES 

from a riser/' * until the connection is frozen up. The 
walls of the casting solidify, but at first are weak and 
yield to the shrinkage of the still fluid iron within the 
casting; if the volume of the casting is large, depressions 
in the walls result. Later the walls become rigid enough 
to resist shrinkage of the remaining fluid within, and 
since the volume cannot be reduced further, portions of 
the mass pull apart and the casting becomes spongy. A 
spongy cross section is necessarily weaker than one of 
solid iron, and is therefore undesirable in a machine 
stress member. Evidently the tendency to form spongy 
iron because of unsupplied fluid shrinkage increases with 
the volume of the casting. 

Experience points to the conclusion that castings of 
small cross section shrink more than those of large cross 
section. To test this conclusion, Mr. Thomas D. West 
made an experiment, which he describes in his book 
" American Foundry Practice." He cast two bars 14 
feet long, from the same iron, and as far as possible made 
the conditions of casting the same for both. The cross 
sections were rectangular, one being 4 inches by 9 inches 
and the other \ inch by 2 inches. The total shrinkage 
for the larger bar was f inch and for the smaller one was 
If inches. This may possibly be explained as follows, 
as Mr. West suggests: A casting cools from the surface, 
and therefore during the cooling the surface will be the 
coolest part, and the heat will increase toward the center. 
The external portions are held from their normal shrinkage 

* A "riser" is formed by making a vertical cylindrical opening 
in the sand which connects with the main portion of the mold. 
The molten iron during pouring rises in this cylindrical opening to 
form the riser. If the riser is large in proportion to the casting it 
remains fluid for some time and acts as a reservoir to supply fluid 
shrinkage. Molten iron from a ladle may be fed in to maintain 
the level in the riser. 



CAST IRON 137 

by the resistance of the hotter internal portions, which 
are not yet ready to shrink as much. This goes on until 
the surface has reached the temperature of the surround- 
ing air and stops shrinking; the hotter portions nearer 
the center now try to shrink as they in turn cool down, 
but are prevented by the external part which has stopped 
shrinking, or it may be that since the thicker portion 
cools more slowly than the thinner portion, it will be grayer 
and the formation of graphite reduces the natural shrink- 
age of the iron. Whatever theory is correct, the fact 
remains that castings of small section shrink more than 
castings of large section. It follows that castings having 
thick and thin parts attached to each other will shrink 
unequally, and be in a state of internal stress, which 
renders them less able to withstand the action of external 
forces. 

Suppose it is required to put a strengthening rib B on A , 
Fig. 26 (a), and that it is made of the form shown, i.e., 
thin relatively to A, and having parallel sides. B would 
shrink more than A, and shrinkage stresses (tension in 
B and compression in A) would result, which would be 
concentrated along the juncture of A and B, and yield- 
ing would occur under a less external force. If the form 
shown in (6) were used, where the rib tapers from the 
thickness of B to the thickness of A, the shrinkage stresses 
would be distributed, and the casting would be stronger. 

The lessons to be learned from these facts are as follows : 
(1) All parts of all cross sections of castings for machine 
members should be as nearly of the same thickness as 
possible, to avoid concentrated shrinkage stresses, with 
their accompanying weakness. (2) If it is necessary to 
have thick and thin parts in the same casting, change of 
form from one to the other should be as gradual as possible. 
(3) Castings should be made as thin as is consistent with 



138 



MATERIALS OF MACHINES 



strength, stiffness and resistance to vibration, to avoid 
the shrinkage stresses, and spongy metal due to the shrink- 
age of large masses. (4) Since some shrinkage stresses 
always must exist in cast machine members, they should 
be taken into account in designing. 

Special care should be taken in the design of wheels, 
because they are peculiarly liable to excessive shrinkage 
stress on account of their form. In a pulley, the thin 
rim tends to shrink more than the heavier arms, and the 
rim is thereby put in tension, and the arms in compression. 
It is not uncommon to see a rim ruptured in this way. 
If the same pulley has a relatively heavy hub, the latter 
will remain fluid until the arms and rim have solidified; 
the tension on the rim will then force the arms into the 
yet fluid hub, which in turn shrinking, will put the arms 
in tension. The arms of fly-wheels tend to shrink away 
from the heavier rim, and are therefore in tension. 

White iron shrinks more than gray iron, and the reason 
is obvious. When graphite is formed from the 7 (C) or 
Fe 3 C a substance of low density replaces a substance of 
high density and the volume of the mass is thereby in- 
creased. 

TABLE OF DENSITIES OF DIFFERENT GRADES OF CAST IRON * 



Grade of iron 


Specific gravity 


Pure iron . 


7.86 
7.60 
7.35 
7.20 
6.80 


White cast iron 


Mottled cast iron 

Light gray cast iron 

Dark gray cast iron 



* Abridged from "The Metallurgy of Iron and Steel 
(McGraw-Hill Book Co.). 



by Bradley Stoughton 



The white castings for mallifying shrink more than gray 
castings; but during mallifying, as a result of the appear- 



CAST IRON 



139 



ance of low-density graphite, the castings expand. The 
resultant shrinkage due to the entire process for production 
of malleable cast iron is about the same as the shrinkage 
of gray castings. 

Effect of internal stress upon the strength of cast- 
ings. — Suppose that a casting is made of the cross- 
sectional form shown in Fig. 26 (a). The part B tends to 
shrink more than A, and, therefore, B is put in tension 
and A is put in compression. Where there is compressive 
stress, and tensile force is applied, the first effect is the 



(a) 



(b) 



Fig. 26. 



reduction of the compressive stress to zero. No tensile 
stress can be induced until the compressive stress is entirely 
neutralized. If a tensile force is applied to the casting 
(a), Fig. 26, it follows that no tensile stress will result in 
the part A, and, therefore, that all the stress will be con- 
centrated on the part B. To illustrate this, suppose that 
a tensile force is applied to a rope, and that half of the 
strands are tight, and the other half are slack. Stress 
will result in the strands which are tight until they are 
strained so much that the others are brought into play, 
and then the tension is sustained by the whole cross 
section, provided the strands originally tight are not 
broken. In the casting, the part B sustains the stress 
until the compression in A is neutralized, and its tensile 
resistance is brought into play. Because of this the unit 
stress (stress per unit of cross-sectional area sustaining 
the stress) is very great in the early part of the test, and 
the deformation, having a proportionate value, is also 




140 MATERIALS OF MACHINES 

much greater than it would be if the whole area of cross 
section sustained the stress. The stress-deformation 
diagram, therefore, takes the form shown in Fig. 27; the 
initial part of the curve representing the concentration 
of stress on some fraction of the cross-sectional area. 
If the stress had been gradually re- 
lieved at A, the curve would have 
returned over AB, and OB would be 
the permanent deformation or "set." 
If the internal stress in B } Fig. 26, 
had been sufficiently great, it might 
have been ruptured before the tensile 
resistance of A could be brought into 
action. In any case, the piece could 
not sustain as great external force as 
if there had been no internal stress, 
because there would be no time during the application of 
force when the whole area of cross section would offer resist- 
ance without some part having been previously weakened. 
A varied quality of product is required from a foundry. 
The most important requirement for some castings that 
are not subjected to any considerable stress is that they 
shall "run sharp "; that is, that they shall take and retain 
the form of the mold accurately. A very gray silicon 
iron with its low shrinkage, and with phosphorus enough 
to give fluidity at casting temperature would serve. 

Other castings require to be as strong as possible in 
tension because, though cast iron is seldom used in direct 
tension, machine members are often subjected to bending 
forces which cause both tensile and compressive stress. 
Still other castings require great compressive strength or 
great compressive shock-resisting capacity; as, for in- 
stance, anvil blocks for power hammers. 

Professor Thomas Turner in a paper in the Trans- 



CAST IRON 



141 



actions of the Iron and Steel Institute 1885, recorded a 
study of all available data to determine the best composi- 
tion of cast iron for given requirements. This study 
seems to indicate that for greatest softness combined 
carbon should equal 0.15 per cent, graphite, 3.1 per cent. 
To obtain this distribution with other substances of 
average values requires 2.5 per cent silicon. For great- 
est general and tensile strength combined carbon should 
equal about 0.5 per cent, graphite from 2.8 to 3 per cent, 
which corresponds to silicon from 1.4 to 1.8 per cent. For 
greatest crushing strength combined carbon should be 
over 1 per cent, graphite under 2.6 per cent, which cor- 
responds to silicon about 0.8 per cent. 

In foundry practice it is desirable to use a large amount 
of "scrap"; partly because "sprues," "gates," "risers," 
etc., are a necessary product of every heat, and partly 
because a good deal of scrap is offered for sale at, a low 
price. The effect of remelting iron is to harden it, and 
therefore, scrap is always of harder grade than the "pig" 
from which it was originally cast. 

The hardening effect of remelting is very clearly shown 
by some experiments made at the Gleiwitz foundry in 
Silesia, and quoted by M. Ferd. Gautier in a paper read 
before the Iron and Steel Institute (see Journal of 1886). 
The results are given in the following table: 



Substances with the iron 


Original pig 
iron 


After fourth 
casting 


After sixth 
casting 


Graphitic carbon 

Combined carbon 

Total carbon 


2.73 

0.66 
3.39 
2.42 
1.09 
0.04 
0.31 


2.54 
0.80 
3.34 
1.88 
0.44 
0.10 
0.30 


2.08 
1.28 
3.36 


Silicon 


1.16 


Manganese 


0.36 


Sulphur 


0.20 


Phosphorus 


0.30 







142 MATERIALS OF MACHINES 

Thus, the six successive meltings resulted in a decrease 
in the amount of silicon and manganese, and an increase 
in the amount of sulphur. (This latter probably was ab- 
sorbed from the fuel.) Graphitic carbon is decreased and 
combined carbon is increased; therefore, the combined 
effect of decrease of silicon and increase of sulphur was 
greater than the effect of the decrease in manganese. The 
change necessary to convert this again into soft gray iron 
is the addition of silicon, provided the amount of sulphur 
is not too great. The reasons for the hardening effect 
of remelting are : (a) the reduction of the silicon, resulting 
in the redistribution of carbon; (b) the increase of sul- 
phur. Of the substances which are found in combination 
with iron, silicon is first oxidized, manganese being next 
in order. Therefore, when iron is melted in the presence 
of an air blast, some of the silicon is always oxidized, 
and usually some of the manganese. Iron is melted in 
the presence of anthracite coal or coke, and hence, there is 
the possibility of absorption of sulphur. If the total 
carbon is sufficiently high, the softening of iron can be 
accomplished very satisfactorily by the addition of a 
proper amount of ferrosilicon, which usually contains 
about 10 per cent of silicon. But if total carbon is low, 
pig iron high in silicon and carbon would serve better, 
because it would carry a large amount of carbon per unit 
of silicon. 

"Burnt scrap" is cast iron which has been exposed 
during use to the action of oxygen at high temperatures; 
as, for instance, old grate-bars, salt-kettles, etc. A por- 
tion of the iron becomes iron oxide. When such iron is 
melted, the iron oxide gives up its oxygen to the silicon, 
manganese or carbon present, in obedience to the law of 
affinities; and the results are silica and oxide of manganese, 
solids which appear as slag, and the gas, carbon monoxide 



CAST IRON 143 

or carbon dioxide. The reduction of the total carbon 
will result in harder iron, and the reduction of the silicon 
will result in the appearance of all the carbon present as 
combined carbon. This result is so very decided that a 
whole heat may "run hard " because of the introduction 
of a comparatively small amount of "burnt scrap." If 
the effect of burnt scrap is due simply to the fact that 
the silicon has been removed by the oxygen of the iron 
oxide, then if it were melted together with a sufficient 
amount of ferrosilicon, the result would be gray, soft iron. 
But there might be iron oxide enough present to reduce 
the total carbon too much; then the silicon could not 
produce gray iron, because it would not have enough 
carbon to work with; in this case, carbon as well as sili- 
con would have to be added, and pig iron high in carbon 
and silicon would serve better than ferrosilicon. The 
iron oxide, which is seen as rust on the surface of scrap, is 
effective in the reduction of silicon, etc., upon melting; 
its effect is of little importance, however, as it is small in 
amount relatively. It must not be concluded from this 
that silicon will make good iron out of all kinds of scrap. 
Some scrap is hopeless because of the presence of sulphur 
or phosphorus. It must be remembered that the addition 
of silicon to very gray iron can produce no good results, 
but rather the reverse, because the carbon is already 
graphitic, and the only effect of the addition of silicon is its 
undesirable effect on the iron itself. 



CHAPTER IX 
STEEL 

It has been shown that steel is essentially a combination 
of iron and carbon, which also contains small amounts 
of silicon, manganese, sulphur and phosphorus. Steel 
also may contain other substances such as copper, nickel, 
chromium, tungsten and vanadium, either from the smelt- 
ing process or introduced because of desirable effect on 
physical properties. 

The chemical difference between cast iron and steel 
is in the amount present of substances other than iron. 
The following table gives a chemical comparison of cast 
iron, the steel used in structures and machines, and tool 
steel. 



Substance 


Cast iron, 
per cent 


Machinery and 

structural steel, 

per cent 


Tool steel, 
per cent 


Carbon 


2.5 to 4.5 
0.15 to 2.5 
to 1.5 
to 0.5 
to 1.3 


0.10 to 0.6 
to 0.04 
0.3 to 1.0 
to 0.06 
0.03 to 0.08 


0.6 to 1.6 


Silicon 


0.04 to 0.25 


Manganese 


0.23 to 0.5 


Sulphur 


0.002 to 0.012 


Phosphorus 


about 0.02 







The change from cast iron to machine steel is effected 
by removing by oxidation all substances other than iron, 
as completely as possible commercially, and then reducing 
iron oxide, removing occluded gas and introducing the 
required amount of carbon. Tool steel is made indirectly 
from cast iron by more complete purification and intro- 

144 



STEEL 145 

duction of a larger proportion of carbon. Some of the 
changes in physical properties that accompany these 
chemical changes may be shown by reference to Fig. 28. 
AD is the stress deformation diagram of cast iron; AB 2 D 2 
of machine steel; ABJ)i of high-carbon or tool steel. 
Change from cast iron to machine steel has increased 

ultimate strength in the ratio r?^ 2 ; it has increased 

A W 
ductility in the ratio ~rw', it has increased ultimate 

shock resistance in the ratio . 2 2 2 ; it has 

area ADE ' 

changed a material that may be easily formed by casting 

into one that is much more difficult to cast and that is 

usually formed by forging. More complete purification 

and increase in carbon would change machine steel into 

tool steel, and strength would be increased in the ratio 

** ; ductility would be reduced in the ratio -iteti 
M2JM2 Ahi2 

ultimate shock resistance would be changed in the ratio 

A T) T\ jp 

. * j}J~ - This last change in shock resistance might be 

ABiD2rj2 

either an increase or a decrease, depending on the amount 
of carbon change. 

The field of steel on the equilibrium diagram, Fig. 29, 
may be assumed as limited on the right by the line JJ\ 
at 1.7 per cent carbon, although in special cases the carbon 
content is higher. Lines of slow cooling of steel may 
now be followed on this diagram from a, d and b as on 
page 113. 

First. Cooling from a. — Solidification begins at 02 and 
is complete at a 3 . The solid 7 (C) then falls in temperature 
until ai is reached, where separation of (3 iron begins and 
continues, causing the point of cooling to follow a x 2V to N. 



146 



MATERIALS OF MACHINES 

§ g 8 3 8 ° 





























cT 






/ 
















/ 
































f 














S 






























































cf 














/ 


/ 














,/ 


















































\ 
















\ 
















\ 






















1 Q 















ccT 


CO 
























STEEL 147 

At N /3 iron changes into a iron and with further cooling 
more of the 7 iron of the remaining 7 (C) changes into a 
iron, causing the point of cooling to follow NO to 0. On 
reaching the mass consists of a + 7 (C) + (probably) 
some Fe 3 C. At the 7 (C) is changed into the eutectic 
— very intimately associated crystals of a iron and Fe 3 C, 
the mixture containing 0.9 per cent carbon — which is 
associated less regularly and less intimately with the a 
iron that has formed from N to 0. Any steel with less 
than 0.9 per cent carbon will have this qualitative com- 
position, but the greater the proportion of carbon the 
greater the proportion of eutectic in the cooled steel. 

Second. Cooling from 5 . — The cooling point moves 
vertically from Oi to 0, where the entire solid mass — 
7 (C) with 0.9 per cent — changes directly into eutectic, 
(a + Fe 3 C)o.9 c- This cooled steel is a homogeneous mass 
of intimately associated small crystals of a iron and Fe 3 C. 

Third. Cooling from b. — The cooling point moves 
through 6 2 and 63 and the mass becomes solid 7 (C) . At 61 
FesC separates and reduction of carbon in 7 (C) causes the 
cooling point to follow biO with continuance of the cement- 
ite separation. On reaching 0, the mass consists of 
7 (C)o.9C a n d Fe 3 C; the 7 (C)o.9c changes into eutectic and 
the cooled mass is made up of eutectic plus Fe 3 C. Slowly 
cooled steel with less than 0.9 per cent carbon consists of 
a plus eutectic in varying proportion. Slowly cooled steel 
with 0.9 per cent carbon consists wholly of eutectic. 
Slowly cooled steel with more than 0.9 per cent carbon 
consists of eutectic plus Fe 3 C in varying proportions. 

Reference now to Fig. 28 shows — since low-carbon 
steel is represented by the diagram AB 2 D 2 E 2 and high- 
carbon steel is represented by the diagram AB1D1E1 — 
that the presence of ferrite, a iron, in large proportion 
corresponds to high ductility and medium tensile strength; 



148 



MATERIALS OF MACHINES 



and that the presence of cementite, Fe 3 C, corresponds to 
high strength and medium ductility. Also, the presence 
of ferrite corresponds to greater softness, while the pres- 
ence of cementite corresponds to greater hardness of the 



mass. 



A 


a 







b 
















2600 
,2400 
2300 




s<*2 


L:c 


kid 

solution 

fOKC 


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:L<iqii 

s 


dana"** 
id Soluti 
\7(C) 


on 


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^^ 


"-^ 






t 
/ 
/ 

/ 




Sol 


d 7(( 


:) 






B 






""-> 


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1800 

1660 

1600 

j3-K 












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/ 

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3s 














K 






/ 


V 
















'(C 


K 1 




/ 


















1420 


L_^ 


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1400 


t «+w 


»X< 


i 




J 2 














1330 
1200 

1000 

800 


k 


! 


: 


















e— a + 


1 

1 


>fe 8 OH 






Pearlite 


! =S Eut 


3ctie = 


:=(a+F 


e 2 C)wi 


th 0.9g 






; 








Aus 
Cem 
Fen 


enite=^ (C) 
entite=fFe 3 C 
ite =c* iron 




carbon 


600 
400 
200 












































































J! 















0.5 0.91 



1.5 



I 2.5 

Fig. 29. 



3.5 



4.5 



STEEL 149 

When carbon in steel is about 0.5 per cent * or more 
the steel has the quality of hardening; that is, on quench- 
ing in cold water from a full red heat the steel becomes 
intensely hard and quite brittle; this hardness may be 
reduced by raising the steel to certain temperatures (the 
process of tempering) that allow stability tendencies to 
approach equilibrium. Thus tools can be made of varying 
hardness suitable for cutting various materials. The 
quality of hardening is intensified by increase in carbon. 
See also page 167. 

While carbon is the chief factor in the control of the 
physical properties of steel, these properties are also 
modified by other substances present. 

Silicon. — There has been much discussion of the 
effect of silicon on steel, and the conclusion seems to be 
that in amounts usually present it is not a seriously inju- 
rious element either to strength or ductility. It is not 
an important question, since with present methods of 
structural and machine steel manufacture silicon is usually 
present only as a trace or as a maximum of 0.04 per cent. 
In steel castings, silicon is often found up to 0.4 per cent 
without effect on ductility, although tensile strength is 
considerably increased; it is here, doubtless, because of its 
capacity to reduce oxides like CO, FeO and Fe 2 3 , formed 
during melting, and thus to make the castings sound and 
homogeneous. In tool steel silicon is often present up 
to 0.25 per cent. But although silicon in relatively small 
amounts in solution in iron has small effect, with increas- 
ing amount the ductility is reduced with increase in brit- 
tleness. The fact that ferrite is very ductile in mild 
steel, less ductile in malleable castings, and very much 

* The property of hardening really appears when carbon equals 
about 0.25 per cent, but the effect which increases with the carbon 
is not of practical importance with carbon less than 0.5 per cent. 



150 MATERIALS OF MACHINES 

less ductile in cast iron is probably due to the increasing 
amount of silicon in solid solution. 

Manganese. — When steel from the Bessemer or open- 
hearth process is ready to pour, manganese is added 
in the form of ferromanganese or spiegeleisen, and its 
function, as already explained, is, like silicon, to reduce 
CO, FeO, Fe 2 3 and to take up free oxygen to form man- 
ganese oxide. This oxide is removed with the slag; but 
a certain amount of manganese goes into solution with 
the iron and in present practice in structural and ma- 
chine steels this amount is from 0.7 to 1 per cent. In 
such amount the manganese prevents cracking of the 
steel when it is worked hot. With manganese up to 0.6 
per cent there seems to be no effect on tensile strength 
or ductility; from 0.6 to 1 per cent the tensile strength is 
increased without change of ductility. In tool steel 
manganese must be kept low, because it tends to cause 
cracking when the steel is quenched in water for purposes 
of hardening. If manganese in steel is increased to 2 per 
cent, the steel is extremely brittle and continues brittle 
up to about 7 per cent when, with further increase of man- 
ganese, strength and ductility both increase until, at 
14 per cent manganese with 0.85 per cent carbon, tensile 
strength becomes about 2.5 times, and ductility about 1.2 
times that of good mild steel. The material can be 
forged hot, but is so hard when cold that it can only be 
machined by grinding. This special manganese steel is 
used for machine parts requiring great resistance to wear 
and to shock. 

Sulphur makes steel " red short"; that is, it causes 
it to crack when rolled hot; it also makes welding difficult. 
The effect of sulphur on steel when cold has been in doubt, 
because of conflicting evidence. Of course, if its produc- 
tion of brittleness when hot has resulted in the steel 



STEEL 151 

cooling with cracks, strength and general fitness for use 
would be less. If the cooled steel is sound the effect of the 
sulphur might depend on whether it was present as iron 
sulphide or manganese sulphide. It seems probable that 
the presence of iron sulphide would tend toward reduced 
ductility and increased hardness, and it also seems prob- 
able that manganese sulphide would have less undesir- 
able effect. It certainly would be desirable to specify 
sulphur as low as possible without hardship to the steel 
manufacturer. 

Phosphorus tends to cause the formation of coarse, 
crystalline structure during cooling of steel.* Because of 
this, or for other reasons, the cooled steel, although its 
static strength may be somewhat increased, yields more 
easily to shocks and is unsafe as structural material, that 
is, it is "cold short." Both sulphur and phosphorus are 
very undesirable in tool steel since the tendency to crack 
either hot or cold may destroy expensive tools during 
or after heat treatment. 

Arsenic and copper are often present in steel, but 
with modern methods of steel-making the amount of 
these substances is almost always far within the harmful 
limit. 

Nickel. — The introduction of nickel into steel up to 
about 8 per cent increases the elastic limit and ultimate 
strength and also slightly increases ductility. Nickel also 
seems to increase the hardening effect of a given amount 
of carbon. Because of this, nickel steel is extensively used 
as armor plate. In certain cases steel with quite low 
carbon has about 3.25 per cent nickel added; this gives 
a ductile steel with a tensile strength of from 60,000 to 

* Possibly this is because phosphorus causes extension of the time 
of cooling and thus gives opportunity for more complete crystal- 
lization. 



152 MATERIALS OF MACHINES 

80,000 pounds per square inch. The armor plates made 
from this steel are " Harvey ized "; that is, they are heated 
to full redness for a long period with the outer surface 
packed with carbonaceous material; the steel absorbs 
carbon, and the surface, to the depth of from 1 inch to 
1J inches, becomes high-carbon steel, which hardens on 
quenching. Thus the body of the plate has the toughness 
and strength — and therefore the shock-resisting capacity 
— of low-carbon nickel steel, while the surface has the 
hardness of hardened high-carbon steel with the intensifi- 
cation of hardness due to the presence of nickel. Nickel 
steel is often used for structural and machine purposes 
in places requiring great strength and shock resistance. 
When welding is necessary nickel should be kept below 
2 per cent. 

Tungsten added to steel renders it hard and brittle 
and its chief use is in special steels for cutting tools. 
See p. 175. 

Chromium. Ferrochrome with wide variation of 
chromium and carbon content is reduced from chrome 
ore in the blast-furnace, or in carbon-lined crucibles, by 
strongly heating oxide of iron and oxide of chromium 
together. Chromium seems to unite with iron in solid 
solution and also in a chemical compound of iron, chro- 
mium and carbon that is exceedingly hard. Chromium 
seems not only to confer hardness of itself but also to 
intensify the hardness due to carbon; it also, as shown 
by careful experiments,* increases the amount of carbon 
that iron can hold. Thus it may cause a threefold in- 
crease in hardening of steel. Because of this chrome steel 
with Cr about 2 to 2.75 per cent and C 0.9 to 1 per cent 
is used in projectiles designed for piercing armor-plates. 

* See "Steel and Iron for Advanced Students" by Hiorns, 
Macmillan, p. 309. 



STEEL 153 

These projectiles require very careful heat treatment. 
Chrome steel is also used for armor-plate, for jaws of 
crushing machines and for other machine parts requiring 
great hardness. 

Vanadium. — Ferro vanadium is obtained from ores con- 
taining vanadium oxide. This oxide, purified by chemical 
processes, is mixed with iron oxide and powdered pure 
aluminum, in a crucible with magnesite lining, and ignited ; 
the aluminum combines with the oxygen of the oxides, and 
the resulting iron and vanadium, at the high temperature 
produced by combustion of the aluminum, combine to 
form ferrovanadium. This is introduced into the steel 
in the ladle after treatment with spiegel or ferromanga- 
nese, and any required content of vanadium can thus be 
obtained. 

Vanadium has a very powerful influence upon the phys- 
ical properties of steel either through its direct influence 
on the iron or through its indirect influence upon the other 
substances present. The elastic limit, ultimate strength 
and shock resistance of steel are very greatly increased 
by the presence of vanadium in amounts between 0.1 
per cent and 0.18 per cent. The steel usually contains 
manganese and chromium in addition to the carbon, and 
frequently nickel also. 

A standard vanadium steel has the following composi- 
tion: 

Per cent 

Carbon 0.25 to 0.3 

Manganese 0.5 

Chromium 1.0 

Vanadium . 0.17 



154 MATERIALS OF MACHINES 

This steel gives values about as follows: 



Heat treatment 


Elastic limit, 
lb. per sq. in. 


Ultimate 

strength, lb. 

per sq. in. 


Elongation 

in 2 inches, 

per cent 


Annealed 

Oil tempered 


65,000 
125,000 

38,000 


90,000 
136,000 

70,000 


28 
18 


For mild carbon steel corre- 
sponding values 


32 



Impact tests also show very distinct gain in shock 
resistance due to the presence of vanadium. 

Vanadium-nickel and vanadium-chrome-nickel steels 
are also used where great strength, lightness and shock 
resistance are prime requirements; the latter steels have 
a percentage range about as follows: 

Per cent 

Carbon 0.25 to 0.45 

Manganese , 0.5 to 0.7 

Nickel 1 to 1.5 

Chromium 0.6 to 0.8 

Vanadium about 0.18. 

There may be internal stresses in forged material, 
similar to those resulting in cast material from unequal 
shrinkage. They are usually the result of working the 
material too cold. To illustrate: when a thin piece of 
ductile material is laid on an anvil and struck with a 
hammer, the piece is made thinner and longer and broader. 
Suppose now that the piece is thick instead of thin, and 
that it receives a blow as before: the influence of the 
blow extends only a little way into the material, and the 
surface is made longer and broader. Since its extension 
is resisted by the part which is uninfluenced by the blow, 
the material at the surface is put in compression, and the 



STEEL 155 

inner portion in tension. The initial part of the stress- 
deformation diagram would be as shown in Fig. 27. If 
the working is done at a red heat, the material is soft 
and weak, and, therefore, yields to the stresses introduced 
by the hammering or rolling, and equilibrium results. 

Effect of lack of homogeneousness of material on 
the stress-deformation diagram. — In the manufacture 
of wrought iron the elements of the piles of "muck-bar " 
or scrap are drawn out in rolling into long lines of crystals, 
which are separated by more or less slag or oxide of iron. 
Since the pile may be made up of bars or scrap of entirely 
different quality, the structure may lack homogeneous- 
ness. This has a tendency to modify the form of stress- 
deformation diagram. Suppose, for example, that a test 
piece of wrought iron has half of its area of cross section 
of a material whose elastic limit is at E 1 , Fig. 30, and that 
the other half of the cross section is of material whose 
elastic limit is at E. Let a constantly 
increasing tensile force be applied to this 
test piece. When the stress reaches the 
value represented by the ordinate E, the 
weaker part of the material begins to yield 
more rapidly than the stronger part, and 
the unit stress on the stronger part is very 
greatly increased, its elastic limit is ex- 
ceeded, it also yields, and the curve takes 
the form shown, running nearly parallel FlG * 30 ' 

to the axis of X until the stress is again distributed over 
the entire surface of the cross section; then the curve rises 
continuously until the maximum stress is reached. Steel 
may also show this irregularity, since different parts of the 
forging may have different elastic limit, because of differ- 
ent heat treatment, different hot working or superficial 
cold working. 




156 MATERIALS OF MACHINES 

Effect of cold working. — When a piece of ductile 
material is strained beyond its elastic limit, the character 
of the material is greatly changed. If, after a short 
interval of rest, it is tested again, its elastic limit and 
elastic resilience will be found to be higher, its tensile 
strength greater and its ductility and ultimate resilience 
less. The stiffness will be but slightly changed, if at all. 
By cold working, i.e., by any means that gives permanent 
set to cold material, the elastic range is increased, the piece 
is made stronger and better able to resist shocks within 
the elastic limit, but less ductile, and less able to resist 
shocks exceeding the elastic limit. These changes are 
shown graphically in Fig. 31. The stress-deformation 
diagram OEABCD is such as would usually result from a 
test of a ductile material, like mild steel or wrought iron. 
On reaching some point, as E h stress is gradually relieved, 
and the curve descends to the X axis at 0±. On reappli- 
cation of tensile force the curve rises along the line 0\Ei 
nearly parallel to OE. The elastic limit is now at E h a 
point much higher than the original elastic limit E. The 
curve then continues, a little higher than it would if the 
stress had not been discontinued, until the maximum 
is reached at H* 

If the force could have been instantly reapplied at O h 
the line GHJ would probably have coincided with ABC, 
because the change is a function of the time of resting, 
after relief of stress. OEABCD may be considered the 
diagram of one material, and O1E1GHJ the diagram of 

* That the maximum strength is increased has been demon- 
strated by Bauschinger. He first broke a long test piece by tensile 
force. It was of uniform cross section, and hence all of its parts 
must have been strained well past the elastic limit. He then broke 
one of the pieces and found increased strength. This was repeated 
six times, and each repetition resulted in increased strength. 



STEEL 



157 



another material. It is as if a new test began at Oi. 
a represent the first diagram, and 
b the second. The elastic range 
of b, represented by OiE h is 
greater than that of a, repre- 
sented by OE. The elastic resili- 
ence of b, represented by the area 
OiEiF h is greater than that of a, 
represented by OEF. Experiment 
has proved that the points B and 
C are not changed in their rela- 
tion to the axis of Y by the relief 
of stress; and therefore the ductil- 
ity of a, represented by OD, is 
greater than the ductility of b, 
represented by OiD. The ulti- 
mate resilience, proportional to 
the total area under the curve, 
is evidently greater in a than in 
b. OiEi is nearly parallel to OE, 
and hence rigidity is nearly the 
same for both. 

If, instead of the almost im- 
mediate reapplication of force, a 
considerable interval of rest had 
been allowed, say twenty-four 
hours, the elastic limit and ulti- 
mate strength would have been still 
further raised, and the diagram 
would be like O^LMN. If stress 
were not discontinued until the 
maximum hadbeennearlyreached, 
the strained material would resem- 
ble a very brittle material. 



Let 



2 


-3 


a 




1° 




ta 

< 

11 * 

\w\vr 






VlJ 


o 
u. 






O 



158 MATERIALS OF MACHINES 

It may be stated as a conclusion warranted by exper- 
iment (see Trans. Am. Soc. Civil Engineers, Vol. XXIV, 
p. 159), that stress of any character beyond the elastic 
limit will render a ductile material stronger and less duc- 
tile under stress of any other character. Annealing 
removes these effects almost completely. The process 
of "cold rolling/ 7 by which shafting is produced, illus- 
trates the alterations of the qualities of ductile material 
due to stress beyond the elastic limit. In this process 
iron is passed cold through highly finished rolls, under 
intense pressure. The rolled piece has its length increased 
and its cross section reduced, and therefore, since the 
material takes a "set," it must be strained by the treat- 
ment past its elastic limit. 

Professor Thurston made a series of tests to determine 
the effect of cold rolling upon iron. His experiments 
show that there results from the process: (a) an increase 
in tensile strength of from 25 to 40 per cent; (b) an ele- 
vation of the elastic limit of from 80 to 125 per cent; 
(c) an increase of elastic resilience of from 300 to 400 per 
cent; (d) a decrease in ductility of about 75 per cent; 
and (e) a decrease of ultimate resilience of about 40 per 
cent. If, therefore, the product of the process is required 
to withstand stress (and especially shock), which cannot 
exceed the elastic limit, it is far better than the untreated 
iron; but if there is a possibility of shock exceeding the 
elastic limit, the unrolled iron might be better. 

The process of "wire-drawing," i.e., reducing the size 
of wire with increased length by drawing it cold through 
dies, produces the same result as cold rolling, the wire 
requiring frequent annealing to restore ductility. 

The effect of repeated stress. — Between the years 
1859 and 1870, A. Wohler planned and executed a series 
of experiments for the Prussian Government, to determine 



STEEL 159 

the laws governing the behavior of metals under repeated 
stress. By means of his machines, forces of known value 
producing tensile, compressive, torsional or transverse 
stress could be applied with indefinite repetition, until 
rupture occurred, or until it was considered proved that 
indefinite repetition of stress could not produce rupture. 
He formulated a law from the experimental work, which 
in substance is as follows: 

Material may be broken by repeated application of a 
force which would fail to produce rupture by a single 
application. The breaking is a function of range of stress; 
and as the recurring stress increases, the range necessary 
to produce rupture decreases. If the stress is reversed, 
the range equals the sum of positive and negative 
stress. 

The experimental work of Wohler was amplified and 
supplemented by Professor Bauschinger of Munich. He 
drew the following conclusions from his experimental 
work: 

(a) "With repeated tensile stresses, whose lower limit 
was zero, and whose upper limit was near the original 
elastic limit, rupture did not occur with from 5 to 16 
million repetitions." He cautions the designer that 
this will not hold for defective material, i.e., a factor of 
safety must still be used for this reason; and that the 
elastic limit of the material must be carefully determined, 
because it may have been artificially raised by cold work- 
ing, in which case it does not accurately represent the 
material. This original elastic limit may be determined 
by testing a piece of the material after careful anneal- 
ing. 

(6) "With often repeated stresses, varying between 
zero and an upper stress, which is in the neighborhood of 
or above the original elastic limit, the latter is raised even 



160 MATERIALS OF MACHINES 

above, often far above, the upper limit of stress, and it 
is raised higher as the number of repetitions of stress 
increases, without, however, a known limiting value L, 
being exceeded." 

(c) ''Repeated stresses, between zero and an upper 
limit below L, do not cause rupture; but if the upper 
limit is above L, rupture will occur after a limited number 
of repetitions." 

From this it would follow that keeping the range of 
repeated stress within the original elastic limit would 
insure safety against rupture with any number of repe- 
titions whatever. But there is a question whether the 
experimenters have proved their case, since they dealt 
necessarily with a finite number of stress-cycles. The 
conclusion that rupture with repeated stress is a function 
of range of stress seems to be sound; but Professor Bau- 
schinger's conclusion that there is a limit of range within 
which there is absolute safety from repeated-stress rup- 
ture seems questionable. A perfectly elastic material 
is one that returns exactly to its initial condition after 
deformation under stress; there is question whether any 
engineering material is perfectly elastic for any range 
whatever, even under small, slowly applied stress. 

There is evidence * that any change of stress in iron 
causes the magnetic and thermo-electric properties to 
change in an irreversible way, and, to quote Professor 
Ewingif " Every variation leaves its mark on the quality 
of the piece; the actual quality at any time is a function 
of all the states of stress in which the piece has previously 
been placed. It can scarcely be doubted that sufficiently 
refined methods of experiment would detect a similar 
want of reversibility in the mechanical effects of stress." 

* See papers by Prof. J. A. Ewing, Phil. Trans., 1885-1886. 
f See his excellent book, "The Strength of Materials," p. 55. 



STEEL 161 

Professor Ewing also cites experiments by Lord Kelvin 
which show that* "repeated changes of stress have a 
cumulative effect in reducing elasticity, while Wohler's 
experiments show that they also have a cumulative effect 
in reducing strength. It may be conjectured that re- 
peated strains induce a change in molecular structure of 
which the fatigue in strength and the fatigue in elasticity 
are two manifestations." 

Annealing restores both original strength and elas- 
ticity; rest restores original elasticity and, as recently 
proved, f restores strength also. 

In the stress-elongation diagram of cast iron, Fig. 18, 
stress was relieved at C and the path of diminishing stress 
is CDE, while the path of reapplied stress is EFC. These 
two paths enclose an area which, on this diagram of force- 
space coordinates, represents work. If the up and down 
paths were identical, there would be no enclosed area and 
the work done by the increasing force would be completely 
restored on relief of stress in the same energy form, and 
there would remain in the material no permanent result 
of the work done; the material would be in exactly the 
same condition before and after the application of force 
and hence would be perfectly elastic. This is what is 
called a reversible process. But if the up -and down 
paths enclose an area, the process is irreversible, the 
material is not perfectly elastic and an amount of work 
proportional to the area fails of return in its original 
energy form; it is converted into heat by resistance of 
the material to molecular change, the heat is radiated 
away and the resulting molecular change remains. Repe- 
tition of this cycle — called the hysteresis cycle — does 

* Ewing's "The Strength of Materials," p. 56. 
f See "Materials of Construction," by Professor George B.- 
Upton, John Wiley & Sons. 



162 MATERIALS OF MACHINES 

more internal work to cause molecular change, and with 
continued repetition the material would be destroyed. 
Wherever there is a hysteresis loop with repeated stress, 
the work of destruction is under way. The length of the 
loop corresponds to the range of stress; the width of 
the loop corresponds to amplitude of molecular displace- 
ment; and the area of the loop, proportional to length 
and width, corresponds to the work done to produce mo- 
lecular change, and hence to the destroying agency. 
Therefore, increase in range of repeated stress increases 
the destroying agency and hence reduces the number 
of cycles to cause fracture. It is found experimentally 
that the width of the loop increases with the number of 
cycles, and hence the destroying agency has an increas- 
ing value. 

Though the ordinary methods of test do not show a 
hysteresis cycle within the so-called elastic limit of steel, 
yet more refined methods disclose such a loop,* and make 
it extremely probable that repeated stress, even within 
the original elastic limit, would cause rupture with a 
sufficient number of repetitions. Ordinarily machines 
grow obsolete or wear out and are discarded long before 
fatigue failure occurs. 

There is another effect of repeated stress that is inde- 
pendent of the effect on molecular structure. If very 
minute flaws exist in the material, or if continuity is 
broken by small particles of foreign substance, the ten- 
dency of repeated stress is to increase the size of the im- 
perfections and a number of these extending micro-flaws 
might join to produce a large crack and eventually to 
cause fracture. Larger hidden flaws might, of course, 
be extended similarly with the same result. 

* The phenomenon known to physicists as " elastic after-effects" 
shows this. 



STEEL 163 

Effect of temperature on steel. Many experiments 
by various careful observers show that when steel is heated 
to about 500° F. its tensile strength begins to decrease, 
and that at the temperature of incipient redness, about 
1000° F., its value is less than half the value at air tem- 
perature. It is also known that prolonged and repeated 
exposure to temperatures of 150° F., or higher, produces 
reduction of ductility, so that to insure safety periodical 
annealing is necessary, as in case of chains of cranes for 
lifting ladles of molten steel or hot ingots. 

Factors of safety. — Machine stress-members may 
fail, not only because of repeated stress, but also because 
of: 

(a) Flaws, or other imperfections in the material; 

(6) Internal stresses; 

(c) Unhomogeneous material; 

(d) Shocks; 

(e) Stresses which cannot be estimated. 

To cover all these a factor of safety is used; i.e., the 
working unit stress is equal to the ultimate unit strength 
of the material, divided by a number which is called the 
factor of safety. 

Materials are so various in their qualities, and the 
conditions to which they are subjected as machine stress 
members are so different, that it is impossible to give any 
value for a factor of safety to cover all cases. 

For ductile resilient material, like mild steel used in 
building-frames, roof -trusses, bridges, etc., a low value 
may be used for the factor of safety, because b, c and d 
given above may be practically eliminated by proper speci- 
fications and careful inspection, and because the loads are 
known. 

But in machines the conditions are dynamic, and it is 
more difficult to estimate stresses, especially when acci- 



164 MATERIALS OF MACHINES 

dental increases of velocity are possible, or when lost 
motion, due to wear or imperfect adjustment, enable 
moving parts to deliver blows to other parts. 

For unresilient or brittle materials, like cast iron, the 
factor of safety needs to be larger, not only because 
of less shock-resisting capacity, but because shrinkage 
stresses are always present and there is, in many cases, 
danger of blow holes or spongy sections. The weaken- 
ing effect of these varies with the size and form of the 
member, and with the conditions of casting. Hence the 
factor of safety must be determined in each case by 
the judgment of the designer. 



CHAPTER X 
HEAT TREATMENT OF STEEL 

When molten steel cools slowly to air temperature its 
structure is coarsely crystalline; the size of the crystals 
increases somewhat with the time of cooling and with the 
amount of carbon present. The steel of a tool-steel 
ingot is coarse, brittle and unfit for service. The steel of 
a mild steel ingot — though the coarse structure is less 
marked — also needs treatment to change structure to 
give required strength and ductility. The finest possible 
structure of steel corresponds to highest strength, duc- 
tility and shock resistance; this structure may be pro- 
duced by heat treatment, which consists of heating and 
cooling through certain temperature ranges and with 
certain rates of temperature change. 

The structure of steel, whatever it may be at air tem- 
perature, is changed to the finest possible crystal size 
when the increasing temperature reaches about 1330° F., 
corresponding to the line MO, Fig. 29. If the increase 
in temperature continues from 1330°, the crystal size grows 
steadily larger until fusion begins and the size thus reached 
is retained, whatever the method of cooling; if, after 
reaching 1330°, the temperature is allowed to fall slowly, 
the crystal size increases until the temperature of dis- 
appearing redness is reached and this size is retained 
independently of the method of cooling. Increase in 
crystal size is undesirable when high shock resistance is 
required, and, therefore, whatever the purpose of heat 

165 



166 MATERIALS OF MACHINES 

treatment it should produce the finest structure possible 
under the circumstances in steel that is required to resist 
shock. 

When points representing cooling steel pass slowly 
through the territory, Fig. 29, bounded above by the line 
KNOJz and below by the line MOJ 2 , the steel changes 
from solid y(C), (austenite) to eutectic (a +Fe 3 C)o.9C 
mixed, according to the carbon content, with excess of 
either ferrite (a iron) or cementite (Fe 3 C). Careful 
observation and reasoning by many eminent metal- 
lurgists seem to show that during this change the steel 
passes through three intermediate states with varying 
physical properties. The names given to steel in these 
successive states are austenite, the original 7 (C), mar- 
tensite, troostite and sorbite. 

Austenite is soft and ductile, with medium strength 
and high shock resistance. 

Martensite is intensely hard, strong under steady 
stress, but with low shock resistance. 

Troostite has medium hardness and is strong, ductile 
and tough. 

Sorbite is nearly as soft as austenite and is strong, 
ductile and tough. 

With slow cooling, austenite would change through this 
series and become sorbite and the change would be com- 
plete. But during this change austenite does not change 
wholly into martensite and then wholly into troostite; 
the changes overlap so that before all austenite has become 
martensite the formation of troostite out of martensite 
has begun and all three are present in varying amounts. 
By the time austenite has disappeared the formation of 
sorbite out of troostite has begun, and martensite, troost- 
ite and sorbite are present in varying amounts; then 



HEAT TREATMENT OF STEEL 167 

martensite and troostite disappear in order, leaving 
sorbite alone. If the cooling could be suddenly checked 
at any temperature and held there to establish equilibrium, 
the steel could be held in the state corresponding to that 
temperature and it would have physical properties de- 
pending on the proportions of steel-carbon forms present. 
Objects of heat treatment are as follows: 

1. To relieve internal stress due to cooling or mechan- 
ical working and to produce a soft steel suitable for ma- 
chining; this is called annealing. 

2. To restore fine grain to steel that has been made 
coarse by overheating; this is also called annealing, or 
sometimes refining. 

3. To produce a very hard steel for cutting edges of 
tools or for wearing surfaces; this is called hardening. 

4. To reduce the hardness produced by the hardening 
process to any desired value and at the same time partially 
to restore ductility and reduce brittleness; this is called 
tempering. 

5. To render stress members of machines tough and 
shock resistant for severe service. This is sometimes 
called toughening. 

6. To raise the elastic-limit so that in case of springs 
there may be large yielding without permanent set; 
this is called spring tempering. 

The list will now be considered in detail. 

1. Annealing. — Stresses due to cooling, or to me- 
chanical working at too low a temperature, may be 
relieved by heating to about 900° F., a temperature just 
below incipient redness, and cooling slowly. The material 
becomes soft enough at this temperature to yield to in- 
ternal stresses and to take a new adjustment in equilib- 
rium. Also, if there is any result of a previous hardening 



168 MATERIALS OF MACHINES 

process, it is entirely removed before reaching 900° F. 
If very great softness is required for machining, the steel 
is cooled very slowly from about 1600° F. This, of course, 
produces coarse grain which must be refined, if necessary, 
for shock resistance, by method 2 or 5. 

2. Annealing or refining. — But if the steel had been 
cooled from some temperature above the line MO, the 
size of its crystal structure would have been enlarged 
beyond the size corresponding to MO; the higher the 
.temperature from which the cooling took place the larger 
the crystal size. This size of crystal with its accompany- 
ing brittleness remains unchanged during cooling and 
during reheating until MO is reached, when it is changed 
quite suddenly and irresistibly to very fine crystal size. 
With further increase of temperature the crystals grow 
as before. In order to fully restore the fine structure 
from the coarse structure due to overheating, it is necessary 
to raise the temperature to the limit set by the line KNO 
so that change to y (C) shall be complete.* Hence a 
higher temperature is necessary to restore low-carbon 
or high-carbon steel than to restore steel with 0.9 per cent 
carbon; in fact with the eutectic proportion, 0.9 per cent 
carbon, it is only necessary to heat to MO.f After heat- 
ing the steel above KNOJz, if it is caused to cool very 
slowly by packing in sand or ashes, the change from aus- 
tenite to sorbite becomes complete, and, although the 
crystal size increases during cooling, yet it is the finest 
structure that fully annealed steel can have. This steel 

* It is sometimes necessary to repeat the reheating to insure 
complete refining. 

t The range of the temperature of change during heating is 
somewhat higher than during cooling and hence the temperature is 
raised in practice from 80° to 90° F. higher than that corresponding 
to the diagram and is held for about fifteen minutes at this tempera- 
ture to insure complete change to y (C). 



HEAT TREATMENT OF STEEL 169 

is thus free from internal stresses, and it is soft * and 
ductile with medium tensile strength. 

3. Hardening. — Consider that steel with 0.9 per 
cent C is raised to a temperature of 1400° F., corresponding 
to h, Fig. 29. After passing above it changes into y (C). 
Suppose now that the steel is " quenched, " that is, 
immersed in agitated cold water. This sudden cooling 
tends to check the change of austenite through marten- 
site and troostite to sorbite. The ordinary methods of 
quenching f cannot hold the steel in the austenite form, 
and in the quenched steel martensite is usually the pre- 
dominating form with a considerable amount of troostite 
and very small amounts of austenite and sorbite. The 
steel, therefore, has the qualities of the dominant mar- 
tensite and is hard enough to scratch glass, and, although 
statically strong, is brittle. Steel fully hardened in this 
way is usually too hard and brittle for service and must 
be further treated by the process of tempering. 

4. Tempering. — If the hardened steel is raised in 
temperature a faint yellow color appears at about 425° F., 
on any portion of its surface that has been polished, and 
at this temperature the tendency to change through the 
a.-m.-t.-s.} series begins to be active and the amount of 
martensite decreases while troostite and sorbite increase, 
with corresponding softening of the steel and with decrease 
in brittleness. 

At 475° F. the surface color of the steel becomes full 
yellow. 

* Though not with the maximum softness of about 1600° cool- 
ing temperature. 

t The rate of cooling depends not only on the quantity and tem- 
perature of the water, but also on the relation of cooling surface to 
volume of the steel; hence thick bars would cool more slowly than 
thin bars. 

X Austenite-martensite-troostite-sorbite series. 



170 MATERIALS OF MACHINES 

At 540° F. the surface color of the steel becomes purple. 

At 560° F. the surface color of the steel becomes bright 
blue. 

At 600° F. the surface color of the steel becomes dark 
blue. 

With these increases in temperature there is accompany- 
ing increase of power of the steel to change through the 
a.-m.-t.-s. series toward sorbite, and if the steel is held 
for a few minutes at a temperature corresponding to any 
one of these colors, there will be a reduction in hardness 
which corresponds to the temperature. 

5. Toughening. — When steel is annealed by heating 
to a temperature slightly above KNOJ 3 and cooling very 
slowly to air temperature the crystal size increases through- 
out the range down to about 1000° F. and with this in- 
crease there is loss of ductility and shock resistance. Let 
the temperature above KNOJ s that gives full transfor- 
mation into 7 (C) be called W, and the temperature of 
incipient redness, about 1000° F., be called V. The 
increase in crystal size and in brittleness occurs between 
W and V; the change from austenite to sorbite with re- 
sulting softness and ductility may occur entirely below V. 
Therefore if steel is heated to W and quenched to V, the 
increase in crystal size will be suppressed, or at least 
greatly diminished; and if the steel is allowed to cool 
slowly from V to air temperature the a.-m.-t.-s. series can 
be completed during the cooling. Both the holding of fine 
grain and the completion of the a.-m.-t.-s. series increase 
strength with nearly constant ductility and hence this 
double process toughens the steel. This process was 
first conceived by Mr. John Coffin and applied to car 
axles at the Cambria Steel Works at Johnstown, Pa., 
with remarkable results. It is now very generally used 



HEAT TREATMENT OF STEEL 171 

by manufacturers for steel that is to be subjected to severe 
and repeated shocks. 

6. Spring tempering. — When steel is heated to W and 
quenched in a medium like oil, which cools it less rapidly 
than water, the a.-m.-t.-s. series is more nearly complete 
because of longer time, and the steel is less hard and more 
ductile; but the martensite is still a potent factor in 
determining the physical properties of the steel and it 
not only hardens and reduces ductility, but it also raises 
the elastic limit and thus gives a wider range of yielding 
within the elastic limit. By regulation of the time of 
quenching and sometimes by subsequent tempering, it is 
possible to control the steel-carbon states so as to give 
the required elastic range for all kinds of spring service 
and also to hold brittleness within necessary limits. 

Hot working of steel. — When steel is heated so that 
its representative point on the equilibrium diagram, 
Fig. 29, is in the territory above KNOJs, the structure 
is coarse, the size of grain being nearly proportional to the 
temperature above MOJ 2 , and the grain size persists 
during cooling to air temperature; hence steel cooled 
from high temperatures is coarse-grained and brittle. 
But if steel at these high temperatures is mechanically 
worked so that its dimensions are changed, as by rolling 
or hammering, the coarse crystals are broken up and the 
grain becomes fine. If the mechanical working ceases 
while the steel is still at high temperature, the crystals 
increase again to the size corresponding to the temperature. 
If, however, the mechanical working continues until the 
temperature is reduced to about 900° F., "black heat," 
the steel will have a very fine grain with corresponding 
physical properties. The coarsely crystalline ingots from 
the crucible steel process are heated and hammered into 
commercial bars, and the hammering continues, the 



172 MATERIALS OF MACHINES 

intensity of blows decreasing with temperature, until black 
heat is reached, and the steel is thus given very fine grain. 

Annealing forgings. — In complex forgings, however, 
it is impossible to work all parts uniformly from forging 
heat to black heat; therefore some parts of the forging 
will cool from high temperature without working and hence 
with coarse grain, while other parts will have the fine 
grain due to careful working. This forging can be given 
a uniform grain by heating uniformly to the temperature 
W and cooling very slowly. This uniform grain, however, 
will not have minimum size since it has had opportunity 
to grow through the temperature range W to V. The 
ideal treatment for this forging if it is to endure severe 
and repeated shock is to heat uniformly to W, to quench 
as suddenly as possible * to V, and then allow slow cooling 
to air temperature. This insures fine grain and comple- 
tion of the a.-m.-t.-s. series, thus giving a maximum of 
toughness and shock resistance. 

Annealing steel castings. — Since steel castings cool 
from a molten state, and since the cooling must be rela- 
tively slow, it follows that they must have a coarse grain. 
Moreover shrinkage stresses are greater in steel castings 
than in cast iron because of the higher casting temperature, 
and hence steel castings should be annealed to relieve 
stress and to refine grain if the best results are to be pro- 
duced in resisting stress and shock. Either the annealing 
or toughening process may be used, but the latter will 
produce better results. 

Case-hardening. — Many steel forms require a very 
hard surface to resist wear or impact, and a tough core to 
resist fracture. Forms of low-carbon steel having the 
required core properties may be case-hardened for such 

* Complex forgings may require a slower cooling medium than 
water, like oil or moving air, to prevent over-stress. 



HEAT TREATMENT OF STEEL 173 

purposes. The forms are packed in carbonaceous mate- 
rial, like wood or bone-charcoal, in boxes from which air 
is excluded, and the temperature is raised to full redness, 
and maintained for a sufficient time to produce the de- 
sired result. Carbon migrates into the steel and goes 
into solid solution at this temperature. The depth of the 
effect and the percentage of carbon depends on the tem- 
perature and the time of exposure. The surface is thus 
converted into high-carbon steel while the core remains 
unchanged. The steel form quenched from the case- 
hardening process has a hardened surface, but since the 
carbonizing temperature is higher than is needed for the 
hardening, the grain will be coarser than is desirable 
and it would be better to cool slowly, reheat to W and 
quench, thus getting the desired hardness with fine grain 
and toughness. Case-hardened steel pieces may be heat 
treated exactly like high-carbon steel; they may be 
annealed, hardened and tempered. 

The process of "Harveyizing " is really case-hardening 
applied to large armor-plates. 

Mild steel is often converted into high-carbon steel at 
its surface by immersion for some time in a molten bath 
of potassium cyanide, KCN, which yields up its carbon 
to the steel. 

High-speed tool steels. — The output of a cutting- 
tool of carbon steel is limited because of the limit to cutting 
speed. The work done in removing metal as chips is 
practically all transformed into heat which raises the 
temperature of the tool, of the chips and of the piece 
from which the chips are cut. This heat is radiated away 
and when equilibrium is established between heat devel- 
opment and heat radiation the cutting tool will have a 
definite temperature. If the cutting speed is increased, 
the work — and hence the heat developed — is increased 



174 MATERIALS OF MACHINES 

proportionately, and the temperature of the cutting tool, 
etc., must rise in order that radiation shall increase to dis- 
pose of the increased heat developed. With a given mate- 
rial to cut, and with given conditions of feed and depth 
of cut, there is, therefore, a definite relation between the 
cutting speed and the temperature of cutting tool; hence 
the output of a cutting tool is a function of the tempera- 
ture that it can endure safely. 

If the temperature of a hardened, tempered carbon- 
steel tool is raised in service above that at which it was 
tempered the temper will be further drawn, the edge will 
be softened and will fail. The limits of temperature for 
such tools is about 450° F. Modern high-speed tool steels 
hold an edge satisfactorily at red heat. 

About 1860-70 Robert Mushet of the Titanic Steel 
Company in England discovered that if steel contained 
tungsten, chromium and manganese, together with high 
carbon and the usual other substances, this alloy cooled 
slowly in air was nearly as hard as carbon steel quenched 
in water. This " Mushet " steel or self-hardening steel 
or air-hardening steel, as it was variously called, was 
used for many years because it increased the possible 
cutting speed — since its temper could not be drawn. 

Messrs. Frederick W. Taylor and Maunsel White of the 
Bethlehem Steel Works carried out a masterly series of 
investigations * which led them to the invention of 
modern high-speed steels and which by increasing the 
cutting efficiency of tool steel from 100 to 200 per cent 
has revolutionized machine shop practice. 

The development of these steels can be shown best by 
reproducing a table from Mr. Taylor's article. This 
table gives the composition and cutting speed of four 

* See Transactions of Am. Soc. Mectil Eng's, Vol. XXVIII, on 
"The Art of Cutting Metals," by F. W. Taylor, p. 31. 



HEAT TREATMENT OF STEEL 



175 



steels that are representative, as Mr. Taylor says, of 
four eras in the development of metal cutting tools. 

The first is Jessop steel which may represent the era 
of carbon-steel tools; the second is Mushet steel, the 
first of the self -hardening steels; the third is the original 
Taylor-White steel, the first of the high-speed steels; 
while the fourth was the best modern high-speed steel in 
1906 when Mr. Taylor's article was written. 





S+= 


a- 




03 


s* 


8 


%■*> 


I 


03 W>_ 

afll 
■+="43 <g 


Kind of steel 


3 a 

1* 


s a 

.£ (J) 

£55 

O 


• 03 
a 

d 

o 

o 


2 « 
a © 

M ,_ 
C 03 
S Q. 


a 9 

73 C3 

is a 

> 


03 
Q. 

a 
8 


IS 




|3 m 
<B O 03 

8/3 S 


Jessop carbon 






1.047 


0.189 




0.206 


0.017 


0.017 


16 




5.44 

8 


0.398 

3.8 


2.15 
1.85 


1.578 
0.3 




1.044 
0.15 


0.025 


0.03 


26 


Original Taylor-White 


58 to 61 


Best modern high 




















speed 1906 


18.91 


5.47 


0.67 


0.11 


0.29 


0.043 






99 











Thus the cutting speed was increased over sixfold. 
Progress from Mushet steel to best modern high-speed 
steel shows very great increase in tungsten and chromium, 
very great decrease in carbon, manganese and silicon, and 
the introduction of the new element vanadium. 

Study of the high-speed steels by the equilibrium dia- 
gram is beyond the scope of this book, but the heat 
treatment necessary for best results is as follows: * 

The cutting end of the tool is first raised slowly and 
uniformly to a bright cherry-red; then it is raised as 
rapidly as possible to a temperature at which the edges 
of the tool begin to fuse; the whole end of the tool must 
be raised uniformly to this temperature of incipient fusion. 

* From Mr. Taylor's paper. 



176 MATERIALS OF MACHINES 

The tool is then plunged into a molten lead bath at a tem- 
perature of 1150° F., where the tool temperature is very 
rapidly reduced to, or below, 1550° F. The amount of 
lead in the bath must be enough so that its temperature 
shall not be sensibly raised by the heat given out by the 
cooling tool, because it is important that the temperature 
of the tool shall not be raised at all, at any time during 
this cooling process. From the temperature of 1550° F., 
or lower, the cooling to air temperature may be fast or 
slow without harmful result. The process thus far is 
called the " high " heat treatment. 

The tool is then given "low " heat treatment as follows: 
it is heated, slowly at first, and then, through the agency 
of a lead bath which is kept at about 1150° F.; to a tem- 
perature that must be above 700° F., and below 1240° F.; 
it is held at this temperature about five minutes and then 
cooled either in an air-blast or simply by exposure to still 
air. The tool is then ready for use and it will hold its 
cutting edge after it has grown red hot under the cut. 
It is important that the temperature shall not exceed 
1240° F. during this low heat treatment because there 
would result great reduction of the quality of " red- 
hardness.' ' 



CHAPTER XI 
NON-FERROUS ALLOYS 

When two metals are melted together, one usually 
takes the other into liquid solution, or perhaps the two 
metals take each other mutually into solution, with wide 
range of composition. The temperature of solidification 
of the preponderating metal is sometimes reduced by the 
introduction of the other metal, as when an increasing 
amount of zinc is added to copper; or the solidification 
temperature may be increased, as when the process is 
reversed and increasing amounts of copper are added to 
zinc. When the cooling alloy solidifies, the solution may 
continue into the solid state; or the alloy constituents 
may separate completely from each other; or chemical 
combination of portions of the constituents may occur; 
or there may be combinations of these results. In most 
of the alloys that are useful to the engineer, the solid is 
composed of one or more crystallized solutions. In some 
cases there are several different possible solutions, and 
the ones that form depend on the proportions of the con- 
stituents present. 

Alloys with copper as chief constituent are most im- 
portant in engineering work. 

The copper-zinc alloys are usually called brass. 

In the upper part of Fig. 32, the brass equilibrium 
diagram * is shown with the entire range from copper 

* This diagram is from a paper on the Constitution of the Copper 
Zinc Alloys by Mr. E. M. Shepherd in the Journal of Physical Chem- 
istry, Vol. VIII, p. 421. 

177 



178 



MATERIALS OF MACHINES 



100 per cent to zinc 100 per cent. In the lower part of 
Fig. 32 are curves that show strength and ductility of the 



2012 




*A.R.C. Alloys Research Committee of the British Institution of Mechanical Engineers. 

Fig. 32. 



NON-FERROUS ALLOYS 179 

alloys of varying composition in the forms of castings, 
worked rods and annealed alloy. 

In the equilibrium diagram the curve of incipient solidi- 
fication consists of six branches, and there are six corre- 
sponding solid solutions or phases of copper and zinc. The 
branches and corresponding phases are as follows: 

Branch Phase 
AB a. 

BC 

CD 7 

DE 8 

EF e 

FG t; 

A liquid solution of zinc in copper, represented by the 
point h on the diagram, may be followed in cooling. On 
reaching hi the solution begins to solidify, beginning with 
the formation of crystals low in zinc and continuing with 
the formation of crystals with steadily increasing zinc con- 
tent until h 2 is reached and solidification is complete. With 
sufficiently slow cooling diffusion probably would produce 
a homogeneous solid solution of a crystals. The result 
would be the same anywhere from A and b 2 (copper 100 
per cent to 70 per cent) except that the proportion of zinc 
would steadily increase and color would change grad- 
ually from copper-red to light yellow at about 90 per cent 
copper, and to dark yellow at b 2 with copper 70 per cent. 

Reference to the lower curves shows that addition of 
zinc to copper up to 30 per cent Zn, producing the a solid 
solution, gives increase in strength and in ductility, and 
hence in shock resistance, whether the resulting brass 
is in the form of castings, annealed brass or worked rods. 
Also any solution between 6 5 and 6 3 (copper 70 per cent 
and 64 per cent) in cooling eventually enters the field 
where a is the stable solid solution, and this range is char- 
acterized by high strength and ductility — and hence 



180 MATERIALS OF MACHINES 

high shock resistance. Between 6 3 and C 3 (copper 64 
per cent to 40 per cent) equilibrium corresponds to a 
mixture of a and 7 solid solutions, and through this range 
there is first a steep rise in strength up to about 53 per 
cent copper and a steep drop in ductility to zero at copper 
about 50 per cent, and then the strength falls very rapidly. 
This change is probably due first to the influence of in- 
creasing a + 7 eutectic and later to increasing amount 
of free 7 solution. From 50 per cent copper to the pure 
zinc end of the range the alloys are worthless to the en- 
gineer. 

Copper-zinc within the range (copper 63 per cent to 
56 per cent) is sometimes called Muntz metal; this alloy 
can be rolled or forged at a red heat, that is, in the field 
BC2C1, where it is in the form of /3 solution; it is used 
for sheathing and fastenings for ships. The lower dia- 
gram shows that with this alloy there is some sacrifice of 
ductility to gain strength. 

For castings the alloy usually contains copper about 
67 per cent, while for ingots to be drawn into tubes or 
wire the copper is about 70 per cent, corresponding nearly 
to maximum ductility. 

Cold working breaks down the crystalline structure of 
brass, increasing strength and brittleness, and during the 
processes of drawing tubes or wire the brass must be fre- 
quently annealed to restore ductility. The effect of 
cold working is obvious on comparison of the tensile 
strength curve of worked rods with the curve of annealed 
brass in Fig. 32. This increase in strength must have 
been accompanied by reduction of ductility. 

The annealing is accomplished by heating the brass to a 
temperature between 1100° F. and 1200° F. where recrys- 
tallization occurs. Quenching from this temperature 
tends to hold crystals small whereas they increase in size 



NON-FERROUS ALLOYS 181 

with slow cooling. With either method of cooling the 
alloy is soft. 

The quality of brass is often affected by the presence of 
substances other than copper and zinc; these substances 
may enter as impurities with one of the constituents, or 
they may be purposely introduced because of their desir- 
able influence of physical properties. 

Aluminum in brass. — In experiments recorded in a 
book on "Alloys," by Sexton,* page 108, aluminum, to 
5 per cent, was added to brass with 60 per cent copper, and 
to brass with 70 per cent copper. The copper content 
was kept constant and the 100 per cent was made up by 
aluminum and zinc. In other words, aluminum displaced 
zinc up to 5 per cent. In both cases the result was an 
increase in tensile strength and a reduction in ductility. 
Since this same result could be accomplished more cheaply 
by increasing the proportion of zinc, the use of aluminum 
would seem hardly to be justified. Of course a small 
amount of aluminum would be useful as a flux in melting 
and casting, reducing copper oxide and removing gas that 
would produce porosity; but this aluminum would not 
appear in the alloy. 

Manganese in brass. — When manganese, to 7 per 
cent, displaces zinc in brass with 60 per cent copper f the 
effect is slight increase in strength and slight decrease in 
ductility. Also there is the same result when manganese, 
to 10 per cent, displaces zinc in brass with 70 per cent 
copper. The result here, as in the case of aluminum, 
would not seem to justify the use of manganese. The 
manganese, like the aluminum, may be used for a flux. 

Iron in brass. — Iron is often present in brass in very 
small amount, derived from iron tools used during melting 

* Scientific Publishing Company, Manchester, 
f " Alloys," by Sexton, p. 117. 



182 MATERIALS OF MACHINES 

and cooling. Iron is also introduced into brass up to a 
little more than 1 per cent giving a ternary alloy called 
" delta metal " which is said to have advantages of 
strength and ductility. The iron is first alloyed with the 
zinc, and this alloy is then melted with copper. 

Arsenic and antimony in very small quantities are 
often present in brass, being brought as impurities in the 
copper; both are very undesirable, making the brass hard 
and brittle. Antimony is especially bad in brass that is 
to be rolled or drawn since it produces " cold-shortness"; 
it should not exceed 0.01 per cent. The ill-effect of 
arsenic is less, but it should not exceed 0.05 per cent. 

Oxygen in brass. — The copper constituent of brass 
oxidizes very readily during melting and casting, and 
copper oxide formed makes the brass weak and brittle. 
As stated above the copper oxide can be reduced by use 
of aluminum or manganese used as a flux; phosphorus also 
is often used very effectively for this purpose. 

Bronzes or copper-tin alloys. — Fig. 33 gives the 
equilibrium diagram * for the copper-tin alloys, with 
the tensile strength and ductility diagram in the same 
relation as for copper-zinc in Fig. 32. The strength and 
ductility curves show that useful alloys for stress-members 
are confined to the range copper 100 per cent to copper 
about 85 per cent. 

The equilibrium diagram shows that several solid 
solutions and one chemical compound are formed in the 
series; the a solution seems to possess fair strength and 
ductility and hence shock resistance; the /3 solution 
obviously is not stable at air temperature, and the presence 
of the free 5 solution is fatal to ductility. With ordinary 

* From a paper " The Constitution of the Copper-Tin Alloys," 
by E. S. Shepherd and E. Blough, Journal of Physical Chemistry, 
Vol. X, p. 630. 



NON-FERROUS ALLOYS 



183 



rate of cooling /3 crystals in small amount are retained 
with copper under 91 per cent and a larger amount may 
be held by quenching. 

The copper-tin alloy that gives the best combination 
of strength and ductility, and hence of shock resistance, 
is copper 90, tin 10, and this alloy is most commonly used 
in machines. 



2192 




Fig. 33. 



184 MATERIALS OF MACHINES 

Heat treatment of bronzes. — Shepherd and Upton 
made a careful series of tests of the physical qualities of 
the copper-tin alloys.* • The strength and ductility 
curves of Fig. 33 are taken from their report. Their 
work also included a study of heat treatment of these 
alloys. The heat treatment methods were as follows: 

A. — Heated to low-red, water quenched. 

B. — Held one week at 1000° F., water quenched. 

C. — Tested as cast. 

D. — Held one week at 752° F., furnace cooled. 

The lettered curves of Fig. 34 show the effect of these 
methods of heat treatment upon the ultimate strength 
of bronzes from copper 100 per cent to copper 65 per cent. 
In Fig. 35 the curves show the corresponding effects upon 
ductility. Methods B and D are of scientific interest, 
but only methods A and C can be practically applied. 
Curves A and C, Fig. 34, show that heating castings to 
redness and quenching has little effect on strength through 
the range copper 100 per cent to 87 per cent; but that 
from copper 87 per cent to 78 per cent, the heat treatment 
causes a distinct increase in strength. Referring to the 
equilibrium diagram Fig. 33 shows that alloys in the a 
field are unaffected by quenching, whereas raising the 
alloys into the a + /3 field and quenching through the 
a + 5 field increases strength. This result is probably 
due to control of proportions of a, /3 and 8. 

Fig. 35 shows that quenching increases ductility through- 
out the entire range copper 100 per cent to 77 per cent. 
This result is probably due to the holding of fine grain 
by quick cooling. 

* See Journal of Physical Chemistry, Vol. IX, No. 6, p. 441, 
June, 1905. 



NON-FERROUS ALLOYS 



185 




Per cent, Copper 
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D. — Held one week at 752° F., furnace cooled. 



186 MATERIALS OF MACHINES 

M. Guillet's conclusions * from his experiments on 
heat treatment of bronzes are as follows : 

1. In the case of alloys containing over 92 per cent of 
copper, the tenacity is slightly increased by quenching 
between 752° F. and 1112° F., and the elongation is sim- 
ilarly affected. 

2. In the case of alloys containing less than 92 per cent 
of copper the tenacity and the elongation increase de- 
cidedly as soon as the quenching temperature exceeds 
932° F. 

3. Maximum strength is reached, whatever the com- 
position of the alloy, at a quenching temperature of about 
1112° F. 

4. Maximum elongation is reached by quenching from 
temperatures which vary with the composition of the alloy. 
With 91 per cent copper, maximum elongation corre- 
sponds to a quenching temperature of 1472° F., while 
with 79 per cent, the maximum elongation corresponds 
to a quenching temperature of 1112° F. 

5. The difference between the tenacity of the cast 
alloy and that of the metal quenched at the most desirable 
temperature is the greater the less the percentage of 
copper. 

Copper-aluminum alloys. — A very careful investiga- 
tion of copper-aluminum alloys was made by the Alloys 
Research Committee of the British Institution of Mechan- 
ical Engineers, reported in the Proceedings, 1907. In the 
summary of conclusions it is stated that the limit of 
industrially serviceable alloys must be placed at 11 per 
cent of aluminum. 

Fig. 36 gives an equilibrium diagram f of copper- 

* See " Alloys " by A. H. Sexton, p. 136. 

t See The Constitution of the Aluminum Bronzes, by B. E. 
Curry, Journal of Physical Chemistry, Vol. XI, p. 425. 



NON-FERROUS ALLOYS . 187 

aluminum and corresponding strength and ductility 
curves from the A.R.C. report. The aluminum limit in 
the diagrams is 15 per cent. 
In the lower diagram: 

Full line A gives strength of sand castings; 

Full line B gives strength of chill castings; 

Full line C gives strength of rolled bars; 

Broken line A gives elongation in 2 inches of sand 

castings; 
Broken line B gives elongation in 2 inches of 

chill castings; 
Broken line C gives elongation in 2 inches of rolled 

bars. 

The very high values of elongation may be accounted for 
in part by the fact that the original length of tested 
portion was 2 inches. The useful range may be divided 
into two parts: aluminum per cent to 8 per cent, and 
aluminum 8 to 11 per cent. In the first division, increase 
in aluminum content causes steady increase in tensile 
strength, and, up to about 7| per cent aluminum, steady 
increase in ductility. In the second division, the strength 
rises more steeply while the ductility falls steeply to a 
value corresponding to great brittleness at 11 per cent 
aluminum. 

The first division gives strong, very ductile, shock- 
resistant alloys. 

The second division gives more desirable alloys where 
ductility should be sacrificed to gain greater strength. 

Comparison of full and broken curves A and B shows 
that chill casting does not give any strength or ductility 
advantage over sand casting up to 8 per cent aluminum, 
but that between 8 and 11 per cent aluminum chill cast- 
ing increases both strength and ductility. Curves C 



188 



MATERIALS OF MACHINES 



show that rolling increases both strength and ductility in 
the first division, but that in the second division rolling 
gives about the same results as chill casting. 

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castings, the chill castings and the rolled bars; they were 
heated to about 1472° F. and cooled slowly; they were 
heated to about 1472° F. and quenched in water. 

Neither slow cooling nor quenching from 1472° F. seems 



NON-FERROUS ALLOYS 189 

to produce very great change in strength or ductility of 
sand castings. 

In case of chilled castings slow cooling from 1472° F. 
has little effect up to 8 per cent alloys, but from 8 per cent 
aluminum to 11 per cent both strength and ductility seem 
to be somewhat reduced. Quenching from 1472° F. seems 
to have practically no effect upon strength and ductility. 

Rolled bars cooled slowly from 1472° F. show increasing 
decrease in strength throughout the entire range, and 
increase in ductility until near the 11 per cent limit. The 
effect of quenching rolled bars from 1472° F. seems to be 
to decrease strength and increase ductility up to 8 per cent 
aluminum and from 8 to 11 per cent aluminum to increase 
strength and to decrease ductility. 

A comparative study of Figs. 32, 33 and 36 shows that 
there is great similarity among the copper-zinc, the copper- 
tin and the copper-aluminum alloys. In each case there 
are two useful ranges; one of medium strength and high 
ductility, and hence of high shock resistance, another of 
rapidly increasing strength and rapidly decreasing duc- 
tility. 

The first range in each case corresponds nearly to the 
field of a solution of the equilibrium diagram, while the 
second range corresponds to the introduction of some 
other solution of the constituents which in increasing pro- 
portion rapidly reduces the alloys to brittleness and 
uselessness: 

Kalchoids. — Dr. Thurston made a very full series of 
experiments on the ternary alloys of copper, tin and zinc, 
which he called kalchoids. He represented the whole field 
of possible combinations of the three metals by an equi- 
lateral triangular area. Many points at equal distances 
from each other were located in this area, and each rep- 
resented an alloy with certain proportions of the three 



190 MATERIALS OF MACHINES 

constituents. Alloys were made corresponding to each 
point, and tested. At each point was erected a piece of 
wire whose height represented the strength of the alloy 
represented by the point. Plastic material was then filled 
in between the wires, and its surface was molded so that 
the points of the wire just showed through. This surface 
represented topographically the varying strength of all 
possible mixtures of copper, tin and zinc, and the alloy 
of maximum strength was thereby located. (See Thurs- 
ton's " Textbook of the Materials of Construction, " 
page 466.) 

Phosphor bronze. — When any alloy containing a 
high percentage of copper is melted in contact with the 
air, there is a strong tendency to form copper oxide, the 
affinity of copper for oxygen being exceedingly strong. 
If the cooled alloy contains copper oxide, it is weak and 
brittle, just as iron containing iron oxide is weak and 
brittle. Copper alloys are usually melted with charcoal 
upon the surface to prevent oxidation, but the prevention 
is not complete. If phosphorus is added to the alloy 
just before pouring, the copper oxide is reduced and phos- 
phoric acid is formed, i.e., the alloy is purified by the 
fluxing action of the phosphorus. This increases both the 
strength and ductility of the alloy. If an excess of phos- 
phorus is added, part of it may combine with the alloy 
and increase its strength and ductility, but the proportion 
of phosphorus should not exceed 0.1 per cent or brittle- 
ness results; it is probable that the chief value of its 
presence is to prevent the formation of oxide of copper 
during remelting. 

Manganese bronze is made either by fusing together 
(a) copper and black oxide of manganese, or (6) copper 
or bronze and ferromanganese. In the first case the 
product is an alloy of copper, manganese and iron, and 



NON-FERROUS ALLOYS 191 

in the second, an alloy of copper, tin, manganese and iron. 
Some of the manganese is effective in removing, or pre- 
venting the formation of, oxide of copper, while the re- 
mainder combines with the copper or bronze to give it 
very greatly increased strength, ductility and toughness. 
A manganese bronze, copper 83.45 per cent, manganese, 
13.48 per cent, iron, 1.24 per cent, has a strength and 
ductility equal to that of open-hearth steel with 0.2 per 
cent carbon. It is much used for marine propeller- 
wheels because it does not corrode easily. 

All of the useful copper alloys are more or less forgeable. 
"Muntz metal," copper 60, zinc 40, is rolled at a red heat 
into plates for sheathing ships, and into forms for bolts 
and other fastenings. It is stronger, cheaper and more 
durable than pure copper. The working of Muntz metal 
at red heat is possible because it is in the j8 field * at that 
temperature, and the /3 solution is ductile. On cooling, 
however, it enters the a + y field losing ductility. Jhe 
effect of cold working upon the copper alloys is similar 
to that upon iron and steel; viz., the strength and hard- 
ness are increased and the ductility is decreased; hence 
the material is more brittle. This will be clear on com- 
paring hard-drawn brass wire with the same wire after 
annealing. 

Brass and bronze of different composition are used for 
journal boxes, but modern practice favors a box of cast 
iron, brass or bronze, with a lining of softer metal, usually 
a white alloy. 

One group of these white alloys is made up of tin with 
small proportions of copper or antimony or both to pro- 
duce strength and hardness. 

Thus " Babbitt metal " consists of tin about 89 per 
cent, copper 2 to 4 per cent and antimony about 7.5 to 
* See Fig. 32. 



192 MATERIALS OF MACHINES 

9 per cent. This is an excellent bearing alloy, but it is 
expensive. 

Another group consists of lead hardened with antimony. 
The antimony varies from 10 per cent to 20 per cent and 
sometimes a small amount of tin is added. This alloy 
is comparatively inexpensive. 



CHAPTER XII 
SELECTION OF MATERIALS FOR MACHINES 

The more important materials used in machine con- 
struction may be brought together as follows : 

1. High-speed steel. 

2. High-carbon steel. 

3. Mild steel, produced by the Bessemer or open- 
hearth process. 

4. Special structural steel. 

5. Wrought iron. 

6. Cast iron. 

7. Malleableized cast iron. 

8. Steel castings. 

9. Brass or bronze. 

10. White metal. This name includes all of the white 
alloys used for lining journal-boxes, etc. 

1. High-speed steel is valuable because of its hardness 
and toughness which fit it for cutting tools for metals, 
and especially because it retains these qualities when 
raised to red heat. This makes very high cutting speed 
possible with corresponding increase in output of metal 
cutting machines. This steel is very expensive because 
of its constituents, its manufacture and its heat treat- 
ment, but the expense is amply justified by the results. 

2. High-carbon steel is still used for a wide range of 
cutting tools and it is valuable because of its quality of 
hardening and tempering. It is also useful for stress 
members where great strength combined with medium 

193 



194 MATERIALS OF MACHINES 

ductility is of prime importance; also for springs because 
of its wide natural or artificial elastic range. 

3. Mild steel by reason of its medium strength, high 
ductility and low price is used in structures and machines 
for all except special service. 

4. Special structural steels containing nickel, chromium, 
vanadium, etc., are useful because their high strength, duc- 
tility and shock resistance fit them for light shock-en- 
during structures like motor cars and aeroplanes, as well 
as for great shock resistance in projectiles and armor 
plates. 

5. Wrought iron has the advantage over mild steel 
that it forges much more easily, probably because of its 
slag content, and hence it is used quite extensively for 
hand-forging. It is also claimed that it is less subject to 
destruction by corrosion and hence it still competes with 
steel for pipes, boiler tubes and similar service. 

6. Cast iron is almost universally used for forms that 
must be shaped by casting, especially where great weight is 
unobjectionable or where great weight is desirable, as in 
fly-wheels, machine beds or frames. When cast forms 
require both strength and lightness, cast iron gives place 
to other material. 

7. Malleable iron is used for cast forms that require 
great shock-resistance — for purposes for which brittle 
cast iron is unsafe. 

8. Steel castings meet the same need as malleable 
cast iron, but the process for production and the nature 
of the material adapts them to much larger machine 
members. 

9. The copper alloys have no advantage over mild 
steel in strength, ductility and shock resistance, and 
hence, since the cost is very much greater there must -be 
some other advantage to lead to their selection for machine 



SELECTION OF MATERIALS 195 

parts. These alloys are much safer against oxidation 
than steel and hence are used for condenser tubes, often 
for feed pipes, for valves of many kinds and for many 
parts of mechanisms subject to corrosion. Moreover these 
alloys have good anti-friction qualities and are used for 
wearing surfaces. 

10. The white alloys are chiefly useful for wearing 
surfaces. The surfaces of machine parts that move over 
each other under pressure are normally separated by a 
film of lubricating material. But under exceptional con- 
ditions the metallic surfaces themselves may come into 
contact; when this occurs the danger of roughening or 
destroying the surfaces depends somewhat upon the excel- 
lence of the surface and kind of material. 

A material may be well adapted for wearing surfaces 
because of (a) hardness, (6) slipperiness, (c) homogeneous- 
ness or (d) because it is partly composed of lubricating 
material. 

Thus, (a) hardened tool-steel is difficult to roughen 
because of its hardness; (b) white metal, though soft, is 
difficult to roughen, because the roughening agent slides 
over the slippery surface; (c) mild steel would have less 
tendency to roughen an engaging surface than wrought 
iron, because the former has a homogeneous surface, 
while the latter carries streaks of gritty cinder; (d) cast 
iron tends to wear smooth rather than rough, because it 
contains graphitic carbon, a lubricating material. 

The ideal for rotating surfaces would be a hardened, 
accurately ground, crucible-steel journal, with its bearing 
lined with high-grade white metal. But here the question 
of cost enters, for the cost of the journal specified includes 
high first cost for the crucible steel, the cost for hard- 
ening, and a cost incident upon the loss of expensive 
parts through cracking in the process of hardening. In 



196 MATERIALS OF MACHINES 

addition to this, an expensive plant is required for the 
hardening of large journals. 

In standard practice mild steel journals are used with 
bearings lined with white metal; but there are often con- 
ditions that lead to the use of other materials. 

Sliding surfaces in machines are often formed upon 
cast-iron members, and the engaging surface is also of 
cast iron. The frictional loss may be reduced by giving 
one surface a white-metal covering. 

To illustrate the selection of materials for machine 
parts, a few typical examples will be discussed. 

The cylinder of a steam-engine, with its ports and 
its connected steam-chest, is of such complicated form 
that it is almost impossible to shape it by forging; or 
if the forging were possible, it would be too expensive. 
The possible materials which may be used for such a 
cylinder are, therefore, only those which are shaped by 
casting. Brass and bronze would have no advantage 
over cast iron, and would cost about ten times as much. 
They are, therefore, out of the question. Steel casting 
might be used, but the first cost of the material would be 
somewhat greater, and the cost of working in the machine- 
shop would be very much greater. Additional strength 
and resilience would be gained, but this is unnecessary, as 
cylinders, even for very high pressures, can be made of 
cast iron, amply strong and resilient, and yet not objec- 
tionably thick. Moreover, cast iron is one of the very 
best possible materials for the wearing surfaces of the 
cylinder and valve-seat. Cylinders subjected to exces- 
sively high pressure, as 300 to 700 pounds per square 
inch, should perhaps be made of steel castings, as, for 
instance, the cylinders of pumps for pipe-lines, or for 
supplying hydraulic machinery. 

The piston-rod of a steam-engine is of mild steel. 



SELECTION OF MATERIALS 197 

The entire force of the steam acting on the piston must 
be transmitted to the cross-head through the piston-rod; 
also, since the effective area of the piston on the crank 
side equals the total area of the piston less the area of 
the rod, and since the effective area needs to be as large 
as possible, the rod should be as small as possible. There 
is always the liability to shocks, and, therefore, since the 
rod must be small and at the same time strong, and must 
also be capable of resisting shocks, a material of high unit 
strength and of high resilience is required. Soft steel 
is the material which combines these qualities. 

A steam-engine cross-head pin is always made much 
larger than is necessary to safely resist shearing, or spring- 
ing by flexure, to insure the maintenance of lubrication; 
cast iron might serve, then, as far as strength and stiffness 
are concerned, and in fact is sometimes used. But there 
is another important consideration: because of the vibra- 
tory motion of the connecting-rod on the pin, there is a 
tendency to wear the pin oval, and when the boxes are 
" keyed up," they will bind when the rod is in its position 
of greatest angularity, if it is properly adjusted when the 
rod is on the center line of the engine. Because of this it 
is desirable to reduce the wear to a minimum, and this 
points to the selection of a hard material. Hardened tool- 
steel might be used, but it is more expensive than soft 
steel or wrought iron, and there is the danger of hidden 
cracks, resulting from the hardening, which may cause 
accident. If soft steel is case-hardened, it will combine 
a hard surface to resist wear with a soft resilient core, 
free from the danger of cracks. Wrought iron case- 
hardened might be used, but wrought iron, because of the 
method of manufacture, has streaks of cinder in its sur- 
face, and lacks the homogeneity of the steel, and is there- 
fore harder to make, and to keep truly cylindrical. It 



198 MATERIALS OF MACHINES 

therefore should not be used where perfection of bearing 
and accuracy of movement are essential. 

The connecting-rod of a steam-engine is subjected to 
the alternate tension and compression resulting from the 
pressure on the piston, and also to a flexure stress due to 
its vibratory motion. These stresses are very severe, 
and there is also liability to shock. The material of the 
rod should be strong and resilient, and soft steel would 
naturally be selected, since it is a forgeable material. 
But there is another important consideration; the rod is 
to be finished, and wrought iron is much more cheaply 
worked in the machine-shop than soft steel, and the 
expense of forging is also much less. The lack of homo- 
geneity is of no importance, as no part of the rod is a 
bearing-surface. Many connecting-rods are made of 
steel casting, and finished by painting. This makes a 
cheaper rod, but there is always the danger of hidden 
defects, like cracks, due to the excessive shrinkage, or 
blow holes, which may weaken the rod enough to cause 
accident. 

The cross-head of a steam-engine is composed of 
two parts : (a) that which serves to transmit the pressure « 
from the piston-rod to the cross-head pin, and (6) that 
which engages with the guide to produce rectilinear mo- 
tion. The stresses on (a) are severe, and there is lia- 
bility to severe shock; hence it must be of strong resilient 
material; the stresses on (b), however, are less, but it 
must be of material which will run well with the guide, 
which is usually of cast iron, being a part of the engine- 
bed. The cross-head may be made of materials as follows : 
(a) may be made of forged wrought iron or soft steel, 
and (6) may be of cast iron bolted to (a), or the whole 
cross-head may be made of cast iron, the part (a) being 
made enough larger than before to be sufficiently strong; 



SELECTION OF MATERIALS 199 

or the cross-head may be made a casting of steel and a 
"shoe " or "gib " of cast iron or brass may be added to 
provide a proper surface to run in contact with the guide. 

The crank-pin of a steam-engine is subjected to the 
same stress as the cross-head pin, and the velocity of 
rubbing surface is very much greater, hence the ten- 
dency to wear is greater. But the tendency to wear "out 
of round " is less and therefore there is less interference 
with the correct adjustment of the boxes; hence there is 
less reason for keeping the wear a minimum; a good 
journal surface is necessary, and soft steel is used without 
case-hardening. 

The main shaft of a steam-engine needs to be strong 
and rigid to resist a combination of severe stresses, i.e., 
the torsional and transverse stress from the connecting- 
rod, and the transverse stress due to the weight of the 
fly-wheel, and the belt tension. It must also afford a 
good journal surface, and for these reasons it is made of 
soft steel. 

The function of the fly-wheel of a steam-engine is to 
adapt a varying effort to a constant resistance, and it 
does this by absorbing and giving out energy periodically 
by virtue of its inertia, which is proportional to its weight; 
it therefore needs, above all things, to be heavy; it also 
needs to be able to resist the bursting tendency of the 
centrifugal force due to its rotation. The most suitable 
material therefore is that which gives the greatest weight 
in the required form, with the required strength, for the 
least money; and cast iron best fulfills these require- 
ments. 

An engine bed or frame, when it is in one piece, is of 
cast iron, and the reasons are obvious: its form is com- 
plex, and could only be produced by casting; weight is 
not objectionable, but rather an advantage, since it 



200 MATERIALS OF MACHINES 

absorbs vibrations; cast iron is amply strong, and affords 
good wearing surfaces for the cross-head guides. Wrought 
iron is used for engine-beds, where vibrations are less 
important, as in the locomotive, and where lightness and 
compactness are very desirable, as in some marine en- 
gines. The beds of some large roll-train and blowing 
engines are built up of wrought and cast iron. 

The journal-bearings, or boxes for the cross-head pin, 
the crank-pin and the journals of the main shaft are 
now usually made of cast iron or brass, with a babbitt- 
metal lining, because good babbitt metal (tin 80, copper 
10, antimony 10) is found to be a better bearing metal 
than brass, i.e., it runs with less tendency to heat; and 
in the case of the cutting out of the surface, the babbitt- 
lined box is far more quickly and cheaply renewed than 
the solid brass box. 

The eccentric and its strap are almost invariably 
made of cast iron, because they are forms which are 
forged with difficulty, and the cast iron affords ample 
strength and excellent wearing surfaces. The eccen- 
tric-rod, on the other hand, would be cumbersome and 
ugly in appearance if it were made of cast iron and given 
sufficient strength. It is a form which may be easily 
either forged or cast, and is made of forged wrought iron 
or steel, or of cast steel, or of malleableized cast iron. 
Rocker-arms, also, when they are used, require to be of a 
resilient material, and when of simple form may be forged 
of wrought iron or steel, and when of more complex form 
may be of malleableized cast iron, or steel casting. The 
valve is usually of somewhat complex form, and needs to 
wear well with the cast-iron valve-seat, and is almost 
invariably of cast iron. 

Considerations similar to those above apply to the 
selection of proper material for the parts of machine 



SELECTION OF MATERIALS 201 

tools. Thus, in the case of a lathe, the bed, legs, head 
and tail-stock, cone, gears, etc., are of cast iron, because 
they are all forms which are most cheaply and satisfac- 
torily produced by casting, and the cast iron affords the 
required strength and stiffness, and satisfactory wearing 
surface, where they are required. Such parts as lead- 
screws, feed-rods and other parts which are subjected to 
considerable stress, and have great length relatively to 
their lateral dimensions, are made necessarily of wrought 
iron or steel. Many of these parts may be finished in the 
machine-shop directly from merchant-bar stock, thus 
saving expense for forging. 

The material for the parts of planing, milling, and 
drilling machines are determined from exactly similar 
considerations. 

Spindles, however, require special attention. In 
lathes, milling and grinding machines the accuracy of 
the work produced depends largely upon the accuracy 
of the spindle. 

The vital point therefore is to maintain this accuracy, 
i.e., to prevent wear as far as possible. It would seem 
then that hardened tool-steel would be the best material. 
But since only a very small amount of stock can be re- 
moved by the grinding machine after the piece is hard- 
ened, the spindle must be roughed out very nearly to size 
before it is hardened; this involves a very considerable 
expense, and there is danger that it may crack in harden- 
ing, or spring so as not to hold up to finish, in which case 
the loss is great, and it is found that the risk cannot be 
taken. The next best thing is to specify machinery steel 
high in carbon (say 0.4 per cent), and to use this harder 
material for the spindle without hardening. In milling- 
machines and in some lathes the main spindle-box is solid, 
of tool-steel, hardened and ground (the risk of loss being 



202 MATERIALS OF MACHINES 

less in this case), and the spindle as before is of 0.4 per cent 
carbon machinery steel. The wear is thus greatly re- 
duced, and the possibility of wear after long use is provided 
against by making the bearing taper, and providing end 
adjustment. The spindles of very large lathes are some- 
times made of cast iron, because forged material would be 
too expensive. The wear is reduced by making the jour- 
nals very large. 

In the steam or hydraulic riveter the main frame 
which supports the cylinder, and carries the guide for the 
moving die, may be of any reasonable size, and therefore 
can be made strong enough to resist even the very great 
forces applied to it, if the material used is cast iron. 
But the " stake," the member which carries the stationary 
die, must resist exactly the same forces as the main frame, 
and must also be small enough so that small boiler-shells, 
and even flues, can be lowered over it to be riveted. The 
" stake " is therefore of forged wrought iron or steel, 
or else a steel casting. 

Suppose that in a machine there is need of a gear and 
pinion whose velocity ratio is 8 to 1, and that the force 
transmitted is large. A tooth of the pinion comes into 
action eight times as often as a tooth of the gear, and 
therefore would wear out in one-eighth of the time if 
both were of the same material; then, too, the form of the 
pinion-tooth in most systems of gearing is such that it is 
much weaker than the gear-tooth. The material for the 
pinion needs, therefore, not only to be stronger, but also 
better able to resist wear. The gear is made of cast iron; 
if the teeth are cut, the pinion may be made of forged 
steel; if the teeth are cast and used without "tooling," 
the pinion may be made a steel casting. 

Material for Springs. — Springs are useful as machine 
parts because of their capacity for yielding without taking 



SELECTION OF MATERIALS 203 

permanent set. The yielding, therefore, must occur with 
stresses that do not exceed the elastic limit. Clearly, 
then, the material with large elastic range, i.e., with high 
elastic limit, is the best material for spring machine- 
members. 

Crucible-steel has the highest normal elastic limit, and 
this limit is raised by hardening and tempering. This is 
the most commonly used material. Untreated mild steel 
may also be used, but with given stress the spring must 
have greater weight than if higher carbon steel were used. 
The steel may have its normal elastic limit artificially 
raised by cold working (cold rolling or wire-drawing), 
and this improves it as a spring material. Brass, bronze 
and other alloys are used for springs, but usually in the 
form of hard-drawn wire with an artificial elastic limit. 



INDEX 



Page 

Acid, neutral or basic lining for furnaces 30 

Alcohol as a solution of the future fuel problem 19 

Alcohol not an important industrial fuel 18 

Aluminum 12, 88 

Aluminum and copper, equilibrium diagram of 188 

Aluminum as a fuel 13 

Aluminum in brass 181 

Annealing 69, 158, 167 

Annealing brass 179 

Annealing forgings 172 

Annealing restores strength and elasticity in steel 161 

Annealing steel castings 172 

Anthracite coal, — chemical changes while in process of for- 
mation 14 

Antimony and arsenic in brass *. 182 

Arc furnace 26 

Arsenic and antimony in brass 182 

Arsenic and copper, presence of, in steel 151 

Artificial fuels 2, 15 

Artificial gas, processes for production of 20 

Artificial modulus of elasticity 103 

Austenite 166 

Babbitt metal 191 

Basic, acid or neutral lining for furnaces 30 

Basic Bessemer process 61 

Basic Bessemer process, graphical representation of 65 

Basic Bessemer process, silicon an undesirable element in ... . 62 

Bauschinger's experiments on repeated stresses 159 

Bauxite 34 

Belgian process for smelting zinc 87 

Bessemer converter, lining of 61 

205 



206 INDEX 

Page 

Bessemer process 12, 58 

Bituminous coal, composition of 15 

Blast-furnace, chemical charges in 42 

Blast-furnace, function of 40 

Blast-furnace stack, continuous flow of gas from 45 

Blast-furnaces, charcoal as fuel for 48 

Blister copper 80 

Brass 177 

Brass, annealing 179 

Brass, antimony and arsenic in 182 

Brass, effect of cold working on 180 

Brass, iron in 181 

Brass, manganese in 181 

Brass, oxygen in 182 

British thermal unit 2 

Brittle material 109 

Brittleness and porosity, how to avoid 74 

Bronzes, Guillet's conclusions on heat treatment of 186 

Bronzes, heat treatment for 184 

Bronzes or copper-tin alloys 182 

"Burnt" scrap iron 142 

Calamine 86 

Calcining ore 38 

Calorific power of carbon monoxide 3 

Calorific power of combustible 2, 3 

Carbon 34 

Carbon and iron, equilibrium diagram of 110, 112 

Carbon between the states of cementite and graphite, im- 
portance of control of 123 

Carbon, chief factor controlling physical properties of steel. . . 149 

Carbon, combined, in cast iron 121 

Carbon, graphite an allotropic form of 122 

Carbon, graphite in cast iron 121 

Carbon, introduction of, into the iron sponge 43 

Carbon, limit of, in solid solution in iron 115 

Carbon monoxide, calorific power of 3 

Carbon monoxide gas, temperature produced by burning. ... 6 

Carbon, removal of, in puddling process 55 

Case-hardening 172 

Cast iron and steel, chemical difference between 144 



INDEX 207 

Page 

Cast iron and steel, temperatures of solidification of 75 

Cast iron, chilling 125 

Cast iron, combined carbon in 121 

Cast iron, composition of 72, 121 

Cast iron, densities of different grades of 139 

Cast iron, graphite carbon in 121 

Cast iron, introduction of sulphur into 43 

Cast iron, malleable, composition of 133 

Cast iron, manganese in 127 

Cast iron, phosphorus in 131 

Cast iron, rate of cooling 125 

Cast iron, silicon in 128 

Cast iron, stress-elongation diagram 101 

Cast iron, structural steel and tool-steel, chemical comparison 

of 144 

Cast iron, sulphur in 127 

Cast iron, two forms of carbon in 121 

Cast iron, white 122 

Cellulose and starch 18 

Cementation process for making tool-steel 57 

Cementite, influence of, on cast iron 121 

Charcoal 15 

Charcoal as a fuel for blast-furnaces 48 

Chemical changes in the blast-furnace 42 

Chemical comparison of structural steel, tool-steel and cast 

iron 144 

Chemical difference between steel and cast iron . . . . : 144 

Chilling cast iron • 125 

Chrome steel 153 

Chromite 33 

Chromium 152 

Coke 15 

Coke, descent of, in blast-furnace 44 

Cold rolling, effect of, on iron, Thurston's experiments 158 

Cold working of steel, effect of 156 

Combination inductive and resistance furnace 29 

Combined carbon in cast iron 121 

Combustible, calorific power of 2, 3 

Combustion, complete 2 

Combustion, heat changes during 5 

Combustion products, nature of 3 



208 INDEX 

Page 

Combustion, temperatures resulting from 3 

Compression 107 

Compressive stress 92 

Connecting-rod of steam-engine, material for 198 

Converter, Bessemer 58 

Cooling cast iron, rate of . . 125 

Copper-aluminum alloys 186 

Copper and aluminum, equilibrium diagram of 188 

Copper and tin, equilibrium diagram of 183 

Copper and zinc, equilibrium diagram of 178 

Copper, oxides, carbonates and silicates of 78 

Copper, sources of 77 

Copper sulphide ores 79 

Copper-tin alloy, best composition for strength and ductility 183 
Copper-tin alloys, Shepherd-Upton tests of physical qualities 

of 182 

Copper-zinc alloy, tensile strength and ductility of 178 

Copper-zinc alloys 177 

Copper-zinc, copper-tin and copper-aluminum alloys, similarity 

of 189 

Crank-pin of steam-engine, material for 199 

Cross-head pin for steam-engine, materials for 197 

Cross-head of steam-engine, material for 198 

Crucible process for making tool-steel 57 

Crucible steel, elastic limits of 203 

Crude petroleum 17 

Cupola furnace, melting pig iron in 50 

Cylinder of steam-engine, materials for 196 

Deformation and stress, simultaneous values of 93 

Deformation proportional to stress within elastic limit 94 

Delta metal 182 

Diagram of cast-iron stress-elongation 101 

Diagram of Diederichs' blast-furnace 46 

Diagram of mild steel stress-deformation 98 

Diederichs' blast-furnace diagram 46 

Diffusion 114 

Dilution, reduction by 68 

Dry blast 48 

Dry-air blast, Gayley's plant for furnishing 49 

Dry puddling 54 



INDEX 209 

Page 

Ductile castings 69 

Ductile material 109 

Ductility 100 

Ductility of steel castings 76 

Duplex process 68 

Eccentric, material for 200 

Eccentric-rod, material for 200 

Eccentric-strap, material for 200 

Elastic deformation 95, 99 

Elastic limit 94 

Elastic limit, artificial 102 

Elastic range 102 

Elastic resilience 99 

Elasticity 100 

Elasticity, definition of 94 

Electric furnaces 26 

Engineering and metallurgical processes, temperatures in ... . 10 

Equilibrium diagram of copper and aluminum 188 

Equilibrium diagram of copper and tin 183 

Equilibrium diagram of copper and zinc 178 

Equilibrium diagram of iron and carbon 110, 112 

Experiments on hardening effect of remelting iron 141 

Factors of safety of brittle materials 164 

Factors of safety of steel 163 

Ferrochrome 152 

Ferrosilicon, use of, in the foundry cupola 129 

Ferrovanadium 153 

Fire-clay 32 

Flaws, repeated stress increases size of 162 

Fly-wheel of steam-engine, material for 199 

Forged material, internal stresses in 154 

Foundry cupola, use of ferrosilicon in 129 

Frame of steam-engine, material for 199 

Franklinite 86 

Fuel, economy of, due to hot blast 47 

Fuels, preliminary consideration of 1 

Galena ore 81 

Galena, roasting 81 

Galena, smelting 82 



210 INDEX 

Page 

Galena, softening 83 

Galena, treatment of 81 

Gas fuel, advantages over solid fuels 19 

Gayley's plant for furnishing dry-air blast 49 

Gear and pinion, material for 202 

Graphite an allotropic form of carbon 122 

Graphite carbon in cast iron 121 

Guillet's conclusions on heat treatment of bronzes 186 

Hall's patent for metallic aluminum 88 

Hardening steel 167, 169 

Harveyized steel 152 

Heat available to raise temperature of products of combustion 7 

Heat treatment for bronzes 184 

Heat treatment for tool-steel 57 

Heat treatment of bronzes, Guillet's conclusions on 186 

Heat treatment of high-speed steels, description of 175 

Heat treatment of steel 165 

High-carbon steel 152 

High-speed tool-steel 173 

Homogeneousness of materials, effect of lack of on stress- 
deformation diagram 155 

Hydrogen, reasons for producing lower temperature than other 

fuels 9 

Hydrogen, temperature produced by combustion of 7 

Illuminating-gas process 20 

Induction electric furnace 28 

Iron, allotropic forms of 110 

Iron and carbon, equilibrium diagram of 110, 112 

Iron, crystal structure of Ill 

Iron, early methods of production 39 

Iron, effect of remelting 141 

Iron in brass 181 

Iron, influence of carbon upon 112 

Iron, limit of carbon in solid solution in 115 

Iron, liquid 112 

Iron ores, composition of 39 

Iron, sources of 38 

Iron sponge 42 

Iron, two forms of shrinkage of 135 



INDEX 211 

Page 

Journal-bearings, material for 200 

Kalchoids 189 

Kaolin 32 

Kelvin's experiments on repeated stresses 161 

Lake Superior copper 77 

Lead 81 

Lime 35 

Lime acting as a flux 42 

Lining of Bessemer converter 61 

Linings for furnaces 30 

Liquid fuels 17 

Low carbon steel 152 

Machine parts, materials for 193 

Machine stress-members, causes of failure of 163 

Machine tool parts, material for 200 

Magnesia 35 

Manganese in brass 181 

Manganese in cast iron 127 

Manganese, introduction of, into iron 43 

Main-shaft of steam-engine, material for 199 

Malleable castings 69 

Malleable castings, analysis of 72 

Malleable castings, tests of 71 

Malleable cast iron, composition of 133 

Manganese bronze 190 

Manganese, function of, in steel 150 

Martensite 166 

Maximum stress 95 

Metal-cutting tools, four eras in development of 175 

Metallic aluminum 88 

Metallic aluminum, Hall's patent for 88 

Metallurgy of copper, lead, tin, zinc and aluminum, outline of 77 

Metallurgy of iron and steel 37 

Mild steel, stress-deformation diagram 98 

Modulus of elasticity 102 

Modulus of elasticity, artificial 103 

Molds for steel castings 75 

Muntz metal 180, 191 

Mushet's self -hardening steel 174 



212 INDEX 

Page 

Native copper 77 

Nickel, introduction of, into steel 152 

Neutral, basic or acid lining for furnaces 30 

Neutral flame 53 

Non-ferrous alloys 177 

Open-hearth furnaces . 73 

Open-hearth processes 66 

Oxidizing flame 53 

Oxygen in brass 182 

Pattinson process for lead 84 

Permanent distortion and breakage 91 

Phosphor bronze 190 

Phosphorus, effect of, on cooling steel 151 

Phosphorus in cast iron 131 

Phosphorus, introduction of, into cast iron 44 

Phosphorus, removal of, in puddling process 55 

Physical properties of steel, carbon chief factor controlling. ... 149 

Pig iron, composition of 40 

Pig iron, composition of, for basic Bessemer process 63 

Pig iron, grayer, from blast-furnace using hot blast 47 

Pig iron, melting, in cupola furnace 50 

Pig iron, uses of 50 

Piston-rod of steam-engine, material for 196 

Planing, milling and drilling machines, material for 201 

Plant tissue 13 

Plastic dolomite for daubing and patching 36 

Porosity and brittleness, how to avoid 74 

Processes for making tool-steel from wrought-iron 56 

Products of combustion, heat available to raise temperature of. 7 

Producer-gas process 22 

Puddling process 51 

Puddling process, removal of carbon in 55 

Puddling, reverberatory furnace for 53 

Pulverized coal 16 

Raw fuels 13 

Reducing flame 53 

Refining process 54 

Refining steel 167, 168 



INDEX 213 

Page 

Refractory materials 30 

Regenerative furnace, Siemen's 66 

Repeated stress, effect of 158 

Repeated stress, range of 160 

Resilience 104 

Resistance electric furnace 27 

Reverberatory furnace for puddling 53 

Riveter frame, material for 202 

Roasting ore 38 

Rocker-arms of engine, material for 200 

Semi-steel 132 

Shearing stress 93 

Shepherd-Upton tests of physical qualities of copper-tin alloys. . 182 

Shrinkage of iron, two forms of 135 

Shrinkage of iron, West's experiment 136 

Siemens-Martin process 67 

Siemen's process 67 

Siemen's regenerative furnace 66 

Silica 33 

Silicon 12 

Silicon an undesirable element in the basic Bessemer process . . 62 

Silicon in cast iron 128 

Silicon, introduction of, into cast iron 43 

Solid fuels 13 

Sorbite 166 

Spiegeleisen 61, 150 

Spindles, material for 201 

Spring tempering 167, 171 

Springs, material for 202 

Squeezer for removing slag *. 55 

Starch and cellulose 18 

Steel and cast iron, chemical difference between 144 

Steel and cast iron, temperatures of solidification of 75 

Steel castings 73 

Steel castings, ductility of - 76 

Steel castings, molds for 75 

Steel castings, shock resistance of 76 

Steel castings, strength of 76 

Steel, effect of cold working of 156 

Steel, effect of temperature on 163 



214 INDEX 

Page 

Steel for castings 75 

Steel, hot working of 171 

Steel ingots, reheating and working 74 

Steel, introduction of nickel into 152 

Steel structure changes with increasing temperature 165 

Stiffness of material 102, 106 

Stoughton converter 73 

Strength at elastic limit 99 

Strength of castings, effect of internal strength on 139 

Stress and deformation, simultaneous values of 93 

Stress and deformation, where proportionality of, ceases 97 

Stress-deformation diagram 97 

Stress within elastic limit, deformation proportional to 94 

Structural steel, tool-steel and cast iron, chemical comparison of 144 

Sulphur, effect of, in steel making 150 

Sulphur in cast iron 127 

Sulphur, introduction of, into iron 43 

Sulphur, removal of, in puddling process 56 

Tar as a binding material 36 

Taylor- White's high-speed steels 174 

Temperature, control of, in Bessemer converter 63 

Temperature, limits of, for high-speed tool-steel 174 

Temperature produced by combustion of hydrogen 7 

Temperatures in engineering and metallurgical processes 10 

Temperatures resulting from combustion 3 

Temperatures of solidification of steel and cast iron 75 

Temper graphite t 69 

Tempering steel 167, 169 

Tensile stress 92 

Testing materials 91 

Thurston's experiments on ternary alloys of copper, tin and zinc 189 

Tin 84 

Tin and copper, equilibrium diagram of 183 

Tool-steel and wrought iron, difference between 56 

Tool-steel, cast iron and structural steel, chemical comparison of 144 

Tool-steel, cementation process for making 57 

Tool-steel, crucible process for making 57 

Tool-steel, sulphur and phosphorus undesirable in 151 

Tool-steels, composition and cutting speed of 175 

Tool-steels, high-speed 173 



INDEX 215 

Page 

" Tossing" process for tin 86 

Total stress 92 

Toughening steel 167, 170 

Troostite 166 

Tropenas converter 73 

Tungsten, addition of, makes steel hard and brittle 152 

Ultimate resilience 99, 104 

Ultimate strength of material 95 

Unit stress 92 

Valves for engine, material for 200 

Vanadium 153 

Vanadium-chrome-nickel steel 154 

Vanadium-nickel steel 154 

Vanadium-steel, composition of ■ 153 

Water-gas process 21 

Wet puddling 54 

White cast iron 122 

Wohler's experiments on repeated stress 159 

Wood, composition of 14 

Wrought iron and tool-steel, difference between 56 

Yield point 97 

Zinc 86 

Zinc and copper, equilibrium diagram of 178 

Zinc, Belgian process for smelting 87 

Zinc blende 86 

Zinc carbonate ore, roasting, to remove moisture . ; 87 





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